Be more compatible with pass(1)

By reading the gpg-id file as well. However, pass(1) supports multiple
gpg-id files maybe we should consider reading all of them.

To increase compatibility further we could also use the same
environment variable names as pass(1):

TPM_STORE_DIR -> PASSWORD_STORE_DIR
TPM_STORE_KEY -> PASSWORD_STORE_KEY
Revert "Set IFS to empty string"

This reverts commit 139f0f3542125dea332b2b38f65d57f32255affc.
Use the same environment variables as pass