platform: Allow custom platform file before install

The new environment variable, PASSWORD_STORE_PLATFORM_FILE is now used
for loading custom platform files while pass lives as
src/password-store.sh. After it is installed using 'make install', this
environment variable is no longer used, and either no platform file is
loaded (if the default platform is acceptable), or a hardcoded
also-installed platform file is referenced.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>