selftests: override `KHDR_INCLUDES` var #50

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 24s
x86_64-llvm-17 / build / build for x86_64 with llvm-17	29m 43s
x86_64-gcc / test (test_maps, false, 360) / test_maps on x86_64 with gcc	4m 2s
x86_64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with gcc	6m 48s
x86_64-gcc / test (test_verifier, false, 360) / test_verifier on x86_64 with gcc	44s
x86_64-gcc / test (test_progs, false, 360) / test_progs on x86_64 with gcc	6m 52s
x86_64-llvm-17 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-17	3m 54s
x86_64-llvm-17 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-17	7m 8s
x86_64-llvm-17 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-17	6m 39s
x86_64-llvm-17 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-17	57s
	1h 31m 11s

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