fix: avoid double CPU prover initialization in multi binary

[fakedev9999]

Summary

• Move ProverClient::from_env() inside the if args.prove block in scripts/prove/bin/multi.rs
• Previously, running multi without --prove would initialize two CPU provers: one unconditionally at line 88 and another inside execute_multi() — wasting startup time
• Now the prover is only created when --prove is actually passed

Test plan

• Run RUST_LOG=info cargo run --bin multi --release -- --start <block> --end <block> --cache (without --prove) and confirm only one "initializing cpu prover" log line appears

[github-actions]

Metric	Value
Batch Start	13,978,002
Batch End	13,978,007
Witness Generation (seconds)	0
Execution Duration (seconds)	21
Total Instruction Count	499,099,286
Oracle Verify Cycles	60,516,558
Derivation Cycles	356,443,547
Block Execution Cycles	7,035,221
Blob Verification Cycles	23,426,260
Total SP1 Gas	775,692,053
Number of Blocks	5
Number of Transactions	5
Ethereum Gas Used	231,150
Cycles per Block	99,819,857
Cycles per Transaction	99,819,857
Transactions per Block	1
Gas Used per Block	46,230
Gas Used per Transaction	46,230
BN Pair Cycles	0
BN Add Cycles	0
BN Mul Cycles	0
KZG Eval Cycles	0
EC Recover Cycles	0
P256 Verify Cycles	0