meta: prevent linux_kernel_headers from being set to `/usr/include` #3844

Job	Run time
Build mlibc (x86_64, mlibc, gcc)	3m 46s
Compile sysdeps (dripos)	2m 3s
Build mlibc (x86_64, mlibc-static, gcc)	3m 2s
Compile sysdeps (lemon)	2m 3s
Build mlibc (x86_64, mlibc-shared, gcc)	2m 48s
Compile sysdeps (aero)	2m 6s
Build mlibc (x86_64, mlibc-ansi-only, gcc)	2m 56s
Compile sysdeps (ironclad)	2m 4s
Build mlibc (x86_64, mlibc-headers-only, gcc)	1m 57s
Compile sysdeps (lyre)	2m 4s
Build mlibc (riscv64, mlibc, gcc)	3m 22s
Compile sysdeps (keyronex)	2m 6s
Build mlibc (riscv64, mlibc-static, gcc)	4m 16s
Compile sysdeps (managarm)	2m 21s
Build mlibc (riscv64, mlibc-shared, gcc)	3m 17s
Compile sysdeps (vinix)	2m 6s
Build mlibc (riscv64, mlibc-ansi-only, gcc)	2m 58s
Build mlibc (riscv64, mlibc-headers-only, gcc)	1m 54s
Build mlibc (aarch64, mlibc, gcc)	3m 41s
Build mlibc (aarch64, mlibc-static, gcc)	3m 26s
Build mlibc (aarch64, mlibc-shared, gcc)	3m 3s
Build mlibc (aarch64, mlibc-ansi-only, gcc)	2m 58s
Build mlibc (aarch64, mlibc-headers-only, gcc)	1m 58s
Build mlibc (x86, mlibc, gcc)	3m 32s
Build mlibc (x86, mlibc-static, gcc)	3m 3s
Build mlibc (x86, mlibc-shared, gcc)	2m 53s
Build mlibc (x86, mlibc-ansi-only, gcc)	2m 52s
Build mlibc (x86, mlibc-headers-only, gcc)	1m 50s
Build mlibc (x86_64, mlibc, clang)	2m 51s
	1h 19m 16s

Provide feedback