src: remove unnecessary `c_str()` conversions in diagnostic messages #3325

Job	Run time
Build slim tarball	2m 20s
x86_64-darwin: with shared libraries	56m 34s
aarch64-linux: with shared libraries	47m 34s
x86_64-linux: with shared libraries	53m 2s
aarch64-darwin: with shared libraries	34m 25s
	3h 13m 55s

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