Add an additional flag protection for the Soroban cost calibration in loadgen #6525

Job	Run time
rust-check-git-rev-deps	18s
fmt	16s
cargo-deny (advisories)	42s
build (gcc, current)	1h 15m 32s
cargo-deny (bans licenses sources)	42s
build (gcc, current, --check-test-tx-meta)	56m 19s
build (gcc, next)	1h 16m 52s
build (gcc, next, --check-test-tx-meta)	59m 44s
build (clang, current)	55m 22s
build (clang, current, --check-test-tx-meta)	47m 47s
build (clang, next)	57m 1s
build (clang, next, --check-test-tx-meta)	49m 42s
complete	1s
	8h 0m 18s

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