Add build-scx-selftests action #77

Job	Run time
x86_64-gcc / build-release	0s
x86_64-llvm-17 / build-release	0s
x86_64-llvm-17 / build / build for x86_64 with llvm-17	29m 33s
x86_64-gcc / build / build for x86_64 with gcc	24m 22s
x86_64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with gcc	6m 29s
x86_64-gcc / test (test_progs, false, 360) / test_progs on x86_64 with gcc	6m 19s
x86_64-gcc / test (test_maps, false, 360) / test_maps on x86_64 with gcc	3m 50s
x86_64-gcc / test (test_verifier, false, 360) / test_verifier on x86_64 with gcc	47s
x86_64-llvm-17 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-17	6m 42s
x86_64-llvm-17 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-17	6m 30s
x86_64-llvm-17 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-17	1m 11s
x86_64-llvm-17 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-17	3m 38s
	1h 29m 21s

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