Use Base16 hash for script path.

Issue #9334 shows that `%` characters on Windows result in invalid
paths, also `/` characters on Linux create invalid paths.

This changes from using base64 to using base16 with the same length
we use for unit-ids.

(cherry picked from commit 97f99171)

# Conflicts:
#	cabal-install/src/Distribution/Client/HashValue.hs
#	cabal-install/src/Distribution/Client/ScriptUtils.hs
#	cabal-testsuite/cabal-testsuite.cabal