fixup??? mingw: allow git.exe to be used instead of the "Git wrapper"

The Meson builds really differ wildly from the regular Git for Windows
builds. For example, the `RUNTIME_PREFIX` is not set, and neither is
`HAVE_WPGMPTR`. However, both of them are required for the assumptions
of t0060.221(MSYSTEM/PATH is adjusted if necessary) to hold.

Let's add the `RUNTIME_PREFIX` prereq to ensure that the test case is
skipped if that is not available. Technically, we also need to add a
prereq for `HAVE_WPGMPTR`, but that would require a lot of work, this
flag is not even close to wired through from the Makefile to
`t/test-lib.sh`, and then there's now also Meson to contend with.
Thanks, but I have enough to do already.

Signed-off-by: Johannes Schindelin <[email protected]>

t/t0060-path-utils.sh

@@ -620,7 +620,7 @@ test_expect_success !VALGRIND,RUNTIME_PREFIX,CAN_EXEC_IN_PWD '%(prefix)/ works'
 	test_cmp expect actual
 '
 
-test_expect_success MINGW 'MSYSTEM/PATH is adjusted if necessary' '
+test_expect_success MINGW,RUNTIME_PREFIX 'MSYSTEM/PATH is adjusted if necessary' '
 	mkdir -p "$HOME"/bin pretend/mingw64/bin \
 		pretend/mingw64/libexec/git-core pretend/usr/bin &&
 	cp "$GIT_EXEC_PATH"/git.exe pretend/mingw64/bin/ &&