Update phst_merge_bazel_lockfiles digest to a8506cd #3743

Job	Run time
Test (7.2.1, ubuntu-latest, gcc)	1m 58s
Test (7.2.1, ubuntu-latest, clang)	2m 58s
Test (7.2.1, macos-latest, clang)	3m 2s
Test (7.2.1, windows-latest, msvc)	6m 0s
Test (7.5.0, ubuntu-latest, gcc)	2m 7s
Test (7.5.0, ubuntu-latest, clang)	2m 24s
Test (7.5.0, macos-latest, clang)	2m 22s
Test (7.5.0, windows-latest, msvc)	5m 34s
Test (8.1.0, ubuntu-latest, gcc)	1m 57s
Test (8.1.0, ubuntu-latest, clang)	2m 7s
Test (8.1.0, macos-latest, clang)	3m 4s
Test (8.1.0, windows-latest, msvc)	7m 51s
Test (latest, ubuntu-latest, gcc)	1m 57s
Test (latest, ubuntu-latest, clang)	2m 28s
Test (latest, macos-latest, clang)	2m 41s
Test (latest, windows-latest, msvc)	12m 50s
	1h 1m 20s

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