Revert "Use "../pass" instead of "../src/password-store.sh""

This reverts commit 76d07ccda38d40ea6679673d60b244538ef870ce.
Revert "Allow 'make pass' so we have version of script for testing."

This reverts commit 32e45b0dd6e17b7aa1aa3a76b49dee845f466ad9.

Conflicts:
	Makefile
Makefile: Clean up other remnants of 'make pass'
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>
platform: Auto-detect local platform file

In fact, if we're running from the source directory, just auto-detect
the platform file in the first place.

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