backup/restore: hardcode MD5 algo for format v2/v3 decryption

Old OpenSSL versions (which were used to encrypt format v2 and v3
backups) defaulted to MD5 for key derivation from the password.

OpenSSL 1.1.0+ in R4.1+ changed that default (not only for encryption
but also for *decryption*) to SHA256:

openssl/openssl@f8547f6#diff-5a32e43870f189e8abf5e1b388115b001c9dadc23e582f28b667738857c237b6

Fixes QubesOS/qubes-issues#7852

qubesadmin/backup/restore.py

@@ -679,6 +679,8 @@ def __run__(self):
                         ["openssl", "enc",
                          "-d",
                          "-" + self.crypto_algorithm,
+                         "-md",
+                         "MD5",
                          "-pass",
                          "pass:" + self.passphrase],
                         stdin=subprocess.PIPE,

qubesadmin/tests/backup/backupcompatibility.py

@@ -1142,7 +1142,8 @@ def handle_v3_file(self, f_name, subdir, stream, compressed=True,
         tar = subprocess.Popen(tar_cmdline, stdout=subprocess.PIPE)
         if encrypted:
             encryptor = subprocess.Popen(
-                ["openssl", "enc", "-e", "-aes-256-cbc", "-pass", "pass:qubes"],
+                ["openssl", "enc", "-e", "-aes-256-cbc",
+                 "-md", "MD5", "-pass", "pass:qubes"],
                 stdin=tar.stdout,
                 stdout=subprocess.PIPE)
             tar.stdout.close()