t7510: use $PWD instead of $(pwd) inside PATH

t7510: use $PWD instead of $(pwd) inside PATH

On Windows, $(pwd) will give us a Windows-style path like "D:/foo".
Putting that into $PATH confuses anybody parsing that variable, since
colon is a separator character in $PATH. Instead, we should use the
Unix-style value we get from $PWD ("/d/foo").

This is similar to the cases fixed by 71dd50472d (t0021, t5615: use $PWD
instead of $(pwd) in PATH-like shell variables, 2016-11-11).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>