Fix regression in canonicalizePath on Windows UNC paths #298

Job	Run time
build (macos-latest, lts-12.26, bytestring-0.11.3.0, filepath-1.4.100.0, unix-2.8.0.0)	7m 1s
build (macos-latest, lts-19.21, bytestring-0.11.3.0, filepath-1.4.100.0, unix-2.8.0.0)	11m 36s
build (ubuntu-latest, 8.4.4, 3.0.0.0, before_prepare() { sed -i.bak /utimensat/d configure.ac; })	4m 21s
build (ubuntu-latest, 8.6.5, 3.0.0.0, before_prepare() { sed -i.bak /utimensat/d configure.ac; })	4m 24s
build (ubuntu-latest, 8.10.7, 3.8.1.0)	4m 40s
build (ubuntu-latest, 9.0.2, 3.8.1.0)	4m 31s
build (ubuntu-latest, 9.2.4, 3.8.1.0)	3m 38s
build (ubuntu-latest, 9.4.3, 3.8.1.0)	4m 10s
build (ubuntu-latest, latest, latest)	1m 48s
build (windows-latest, lts-12.26, bytestring-0.11.3.0, filepath-1.4.100.0, time-1.8.0.2, Win32-2....	9m 0s
build (windows-latest, lts-17.5, bytestring-0.11.3.0, filepath-1.4.100.0, time-1.9.3, Win32-2.13....	11m 35s
build (windows-latest, lts-22.7, bytestring-0.11.5.3, filepath-1.4.100.0, time-1.12.2, Win32-2.14...	8m 42s
	1h 15m 26s

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