Rename `c_src` to `cpp_src` #2

Job	Run time
freebsd / build (14.3, aarch64, arm64, freebsd-arm64)	9m 33s
ubuntu / build (arm64, ubuntu-24.04-arm64)	1m 20s
macos / build (macos-15-intel, amd64, macos-amd64)	1m 36s
freebsd / build (14.3, amd64, amd64, freebsd-amd64)	5m 16s
windows / build (x64, amd64)	2m 4s
windows / build (ARM64, arm64)	2m 28s
windows / build (Win32, i386)	1m 46s
ubuntu / build (amd64, ubuntu-24.04-amd64)	58s
macos / build (macos-latest, arm64, macos-arm64)	53s
update-libs	8s
	26m 2s

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