cli: linux: read KOPIA_USE_KEYRING env (#1337)
cli: linux: read KOPIA_USE_KEYRING env (#1337)

On Linux, Keyring is disabled by default and `--use-keyring` must
be specified on every command-line invocation, which gets quite
annoying.

This commit allows enabling keyring from environment variable
instead.
kopia/kopia,kopia/kopia,kopia/kopia,kopia/kopia