Add build-scx-selftests action #73

Job	Run time
x86_64-gcc / build-release	0s
x86_64-llvm-17 / build-release	0s
x86_64-gcc / build / build for x86_64 with gcc	24m 52s
x86_64-llvm-17 / build / build for x86_64 with llvm-17	31m 0s
x86_64-gcc / test (test_verifier, false, 360) / test_verifier on x86_64 with gcc	46s
x86_64-gcc / test (test_maps, false, 360) / test_maps on x86_64 with gcc	4m 7s
x86_64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with gcc	5m 59s
x86_64-gcc / test (test_progs, false, 360) / test_progs on x86_64 with gcc	6m 23s
x86_64-llvm-17 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-17	5m 50s
x86_64-llvm-17 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-17	4m 21s
x86_64-llvm-17 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-17	1m 4s
x86_64-llvm-17 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-17	6m 3s
	1h 30m 25s

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