build(cmake): fail fast with clear diagnostic when .venv has no pip (5.1)

[DonLakeFlyer]

Cherry-pick of #14784 to Stable_V5.1.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (Stable_V5.1@249ac77). Learn more about missing BASE report.

Additional details and impacted files

@@              Coverage Diff               @@
##             Stable_V5.1   #14802   +/-   ##
==============================================
  Coverage               ?   32.68%           
==============================================
  Files                  ?      784           
  Lines                  ?    67602           
  Branches               ?    31311           
==============================================
  Hits                   ?    22093           
  Misses                 ?    30632           
  Partials               ?    14877           

Flag	Coverage Δ
unittests	32.68% <ø> (?)

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[github-actions]

Build Results

Platform Status

Platform	Status	Details
Linux	Passed	View
Windows	Passed	View
MacOS	Passed	View
Android	Passed	View

All builds passed.

Pre-commit

Check	Status	Details
pre-commit	Failed (non-blocking)	View

Pre-commit hooks: 0 passed, 0 failed, 0 skipped.

Test Results

linux-coverage-integration: 37 passed, 0 skipped
linux-coverage-unit: 119 passed, 0 skipped
linux-sanitizers-integration: 20 passed, 0 skipped
linux-sanitizers-unit: 119 passed, 0 skipped
Total: 295 passed, 0 skipped

Code Coverage

Coverage: 69.4%

No baseline available for comparison

Artifact Sizes

Artifact	Size
QGroundControl	221.78 MB
QGroundControl-aarch64	179.35 MB
QGroundControl-installer-AMD64	137.85 MB
QGroundControl-installer-AMD64-ARM64	80.38 MB
QGroundControl-installer-ARM64	109.14 MB
QGroundControl-linux	87.04 MB
QGroundControl-mac	87.04 MB
QGroundControl-x86_64	191.59 MB
No baseline available for comparison

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_{Updated: 2026-08-09 17:06:42 UTC • Commit: bf63c70 • Triggered by: Android}