Fix Windows UNC paths for file encryption

With Qt 5.13.2 (and still there in 5.15.0)
there was a regression that QTemporaryFile on Windows
UNC drives (unmounted Samba shares) have a bad filename even
though the file is correctly created. As the fix in Qt for
this is not trivial this adds a workaround to fix up the path name
and can probably be removed in a future version.

GnuPG-Bug-Id: T5216
2 jobs for master in 11 minutes and 50 seconds (queued for 4 seconds)
Status Job ID Name Coverage
failed #88916


passed #88915
linux kf5-qt5 qt5.15


Name Stage Failure
build_clazy_clang_tidy Build
make[2]: *** [src/libkleopatraclient/core/CMakeFiles/kleopatraclientcore.dir/build.make:196: bin/] Error 1
make[1]: *** [CMakeFiles/Makefile2:599: src/libkleopatraclient/core/CMakeFiles/kleopatraclientcore.dir/all] Error 2
make: *** [Makefile:160: all] Error 2
Uploading artifacts for failed job
Uploading artifacts...
WARNING: JUnitTestResults.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1