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>