miri-script: Transform Windows paths to unix.

python3 snippet used to fill $MIRIDIR variable returns native paths.
Down the line in `bench` subcommand this leads to benchmark setup
failure, preventing contributors from running benchmarks on Windows
hosts.

This commit replaces usage of native os.path module with pathlib, which
explicitly converts paths to Unix flavour.

miri

@@ -77,7 +77,7 @@ if [ -z "$COMMAND" ]; then
 fi
 shift
 # macOS does not have a useful readlink/realpath so we have to use Python instead...
-MIRIDIR=$(python3 -c 'import os, sys; print(os.path.dirname(os.path.realpath(sys.argv[1])))' "$0")
+MIRIDIR=$(python3 -c 'import pathlib, sys; print(pathlib.Path(sys.argv[1]).resolve().parent.as_posix())' "$0")
 # Used for rustc syncs.
 JOSH_FILTER=":rev(75dd959a3a40eb5b4574f8d2e23aa6efbeb33573:prefix=src/tools/miri):/src/tools/miri"
 # Needed for `./miri bench`.