Cargo: configure cargo-c to use vendored .h #1420

Job	Run time
cargo-c	43s
Clang Tidy	6s
Clippy	16s
Build+test (clang, stable, ubuntu-latest)	1m 5s
Clippy nightly (optional)	19s
Build+test (clang, beta, ubuntu-latest)	59s
ensure-header-updated	1m 9s
Build+test (clang, nightly, ubuntu-latest)	1m 1s
Check for documentation errors	24s
Build+test (clang, 1.61.0, ubuntu-latest)	1m 49s
Format	31s
Build+test (gcc, stable, ubuntu-latest)	48s
Check minimum versions	38s
Build+test (gcc, beta, ubuntu-latest)	55s
Miri	1m 17s
Windows	1m 40s
Build+test (gcc, nightly, ubuntu-latest)	47s
Build+test (gcc, 1.61.0, ubuntu-latest)	2m 13s
Windows CMake, Debug configuration	2m 14s
Build+test (macos-latest, clang, stable)	1m 18s
Windows CMake, Release configuration	2m 42s
Valgrind	59s
	23m 53s

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