Add build-scx-selftests action #72

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	23m 17s
x86_64-llvm-17 / build / build for x86_64 with llvm-17	29m 44s
x86_64-gcc / test (test_maps, false, 360) / test_maps on x86_64 with gcc	3m 39s
x86_64-gcc / test (test_verifier, false, 360) / test_verifier on x86_64 with gcc	1m 16s
x86_64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with gcc	6m 9s
x86_64-gcc / test (test_progs, false, 360) / test_progs on x86_64 with gcc	6m 21s
x86_64-llvm-17 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-17	6m 20s
x86_64-llvm-17 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-17	5m 52s
x86_64-llvm-17 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-17	1m 0s
x86_64-llvm-17 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-17	3m 57s
	1h 27m 35s

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