Make secret unlocker add pgp work on Linux (closes #88)

CreatePGPUnlocker got the vault's long-term key from the keychain
unlocker's helper, which on every platform but macOS is a stub that
always fails. It now calls the vault's GetOrDeriveLongTermKey, as adding
a passphrase unlocker does: from the mnemonic, checked against the
vault, or else from the current unlocker. That method joins
VaultInterface. The test GPG key gains an encryption subkey, and a new
test adds a PGP unlocker with the long-term key from the mnemonic and
from a passphrase unlocker, then reads a secret through it.

Model: opus-5-5

internal/secret/pgpunlocker_test.go

@@ -40,9 +40,7 @@ func installFakeGPG(t *testing.T) {
 // TestCreatePGPUnlockerFailureWritesNothing makes CreatePGPUnlocker fail at
 // getting the vault's long-term key, which used to come after part of the
 // unlocker was written, and asserts that nothing is written. Getting the key
-// fails because on macOS there is no mnemonic and no current unlocker, and
-// on every other platform it always fails
-// (https://git.eeqj.de/sneak/secret/issues/88).
+// fails because there is no mnemonic and no current unlocker.
 func TestCreatePGPUnlockerFailureWritesNothing(t *testing.T) {
 	installFakeGPG(t)
 	t.Setenv(secret.EnvMnemonic, "")