Add build selftests/sched_ext step #699

Job	Run time
set-matrix	2s
s390x-gcc / build-release	0s
aarch64-gcc / build-release	0s
x86_64-gcc / build-release	0s
aarch64-gcc / build / build for aarch64 with gcc	4m 55s
x86_64-gcc / build / build for x86_64 with gcc	3m 51s
s390x-gcc / build / build for s390x with gcc	4m 46s
x86_64-llvm-17 / build / build for x86_64 with llvm-17	3m 21s
x86_64-llvm-17 / build-release / build for x86_64 with llvm-17-O2	3m 20s
x86_64-llvm-18 / build / build for x86_64 with llvm-18	3m 19s
x86_64-llvm-18 / build-release / build for x86_64 with llvm-18-O2	2m 31s
aarch64-gcc / veristat	0s
aarch64-gcc / test (test_progs, false, 360) / test_progs on aarch64 with gcc	56s
aarch64-gcc / test (test_maps, false, 360) / test_maps on aarch64 with gcc	55s
aarch64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on aarch64 with gcc	55s
aarch64-gcc / test (test_verifier, false, 360) / test_verifier on aarch64 with gcc	23s
s390x-gcc / veristat	1s
s390x-gcc / test	1s
x86_64-gcc / test	1s
x86_64-gcc / veristat	1s
x86_64-llvm-18 / test	1s
x86_64-llvm-18 / veristat	1s
x86_64-llvm-17 / veristat	1s
x86_64-llvm-17 / test	1s
	29m 22s

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