Carry the vcpkg root under a name vcvars does not own

The six Windows jobs installed Boost into one vcpkg and then pointed CMake at
another. vcvars sets VCPKG_ROOT to the copy Visual Studio bundles, and the
msvc-dev-cmd step sits between the install and the configure, so the variable
the install step wrote was gone by the time the toolchain file and the prefix
path were built from it. The step logs show it outright: VCPKG_ROOT is
C:/vcpkg while boost-test is installing, and the VC\vcpkg under Visual Studio
18 by the time cmake runs.

Visual Studio 2022 does not do this, which is why it never showed anywhere but
on the runner.

The root now travels as STDC_VCPKG_ROOT, which nothing else writes.

This is also the correction to the previous commit. Caching the install state
rather than the built binaries is a real hazard and the change stands, but it
was not what made find_package come up empty: the diagnostic it added shows
BoostConfig.cmake present in C:/vcpkg the whole time.

Author: SineStriker

.github/workflows/ci.yml

@@ -71,10 +71,13 @@ jobs:
         if: runner.os == 'macOS'
         run: brew install ninja boost
 
-      # Ninja is not on the Windows image, and there are two vcpkgs: the one Visual Studio bundles,
-      # which is what VCPKG_INSTALLATION_ROOT points at, and the one on PATH. Deriving the root
-      # from the executable that will actually run the install is what keeps those from being
-      # different directories.
+      # Ninja is not on the Windows image, and there are two vcpkgs: the one Visual Studio bundles
+      # and the one on PATH. Deriving the root from the executable that will actually run the
+      # install is what keeps those from being different directories.
+      #
+      # It is carried under our own name, not VCPKG_ROOT. vcvars sets VCPKG_ROOT to the copy
+      # Visual Studio bundles, so the msvc-dev-cmd step below silently repoints it between the
+      # install and the configure: Boost lands in one vcpkg and CMake is aimed at the other.
       - name: Install Ninja and locate vcpkg (Windows)
         id: vcpkg
         if: runner.os == 'Windows'
@@ -83,7 +86,7 @@ jobs:
           choco install ninja -y --no-progress
           root=$(cygpath -m "$(dirname "$(command -v vcpkg)")")
           echo "vcpkg root: $root"
-          echo "VCPKG_ROOT=$root" >> "$GITHUB_ENV"
+          echo "STDC_VCPKG_ROOT=$root" >> "$GITHUB_ENV"
           echo "VCPKG_DEFAULT_BINARY_CACHE=$RUNNER_TEMP\\vcpkg-archives" >> "$GITHUB_ENV"
           mkdir -p "$RUNNER_TEMP/vcpkg-archives"
           # The port versions come from whichever vcpkg the image shipped, so its commit is what
@@ -112,8 +115,8 @@ jobs:
           # find_package(Boost CONFIG) turns on exactly one file, which the boost-cmake port
           # brings in as a dependency of boost-test. Saying whether it arrived makes a failure
           # downstream one question rather than two.
-          config="$VCPKG_ROOT/installed/x64-windows/share/boost/BoostConfig.cmake"
-          ls -l "$config" || { echo "missing: $config"; ls "$VCPKG_ROOT/installed" || true; }
+          config="$STDC_VCPKG_ROOT/installed/x64-windows/share/boost/BoostConfig.cmake"
+          ls -l "$config" || { echo "missing: $config"; ls "$STDC_VCPKG_ROOT/installed" || true; }
 
       # Ninja everywhere, which needs the MSVC environment on Windows since there is no generator
       # to set it up.
@@ -134,9 +137,9 @@ jobs:
           if [ "${{ runner.os }}" = "Windows" ]; then
             # The prefix path as well as the toolchain file. The toolchain works out the triplet
             # for itself, and naming the directory outright is one fewer thing to be wrong.
-            extra=(-DCMAKE_TOOLCHAIN_FILE="$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake"
+            extra=(-DCMAKE_TOOLCHAIN_FILE="$STDC_VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake"
                    -DVCPKG_TARGET_TRIPLET=x64-windows
-                   -DCMAKE_PREFIX_PATH="$VCPKG_ROOT/installed/x64-windows")
+                   -DCMAKE_PREFIX_PATH="$STDC_VCPKG_ROOT/installed/x64-windows")
           fi
           cmake -G Ninja -S . -B build \
             -DCMAKE_BUILD_TYPE=${{ steps.cfg.outputs.type }} \