src: remove unnecessary `c_str()` conversions in diagnostic messages #3376

Job	Run time
Build slim tarball	2m 28s
x86_64-darwin: with shared libraries	47m 22s
x86_64-linux: with shared libraries	34m 32s
aarch64-linux: with shared libraries	28m 36s
aarch64-darwin: with shared libraries	33m 30s
	2h 26m 28s

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