tests(gpg): allow the gpg-agent to start on Windows

tests(gpg): allow the gpg-agent to start on Windows

In Git for Windows' SDK, we use the MSYS2 version of OpenSSH, meaning
that the `gpg-agent` will fail horribly when being passed a `--homedir`
that contains colons.

Previously, we did pass the Windows version of the absolute path,
though, which starts in the drive letter followed by, you guessed it, a
colon.

Let's use the same trick found elsewhere in our test suite where `$PWD`
is used to refer to the pseudo-Unix path (which works only within the
MSYS2 Bash/OpenSSH/Perl/etc, as opposed to `$(pwd)` which refers to the
Windows path that `git.exe` understands, too).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>