t/lib-gpg: fix setup of GNUPGHOME in MinGW

t/lib-gpg: fix setup of GNUPGHOME in MinGW

In "t/lib-gpg.sh" we set up the "GNUPGHOME" environment variable to
point to a test-specific directory. This is done by using "$PWD/gpghome"
as value, where "$PWD" is the current test's trash directory.

This is broken for MinGW though because "$PWD" will use Windows-style
paths that contain drive letters. What we really want in this context is
a Unix-style path, which we can get by using `$(pwd)` instead. It is
somewhat puzzling that nobody ever hit this issue, but it may easily be
that nobody ever tests on Windows with GnuPG installed, which would make
us skip those tests.

Adapt the code accordingly to fix tests using this library.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>