Fix PEM header for generated public and private keys.

Omiting the key type in the PEM header of the generated key leaves the
key files malformed which confuses some parsers including openssl.

pkg/provisioning/bootguard/keygen.go

@@ -102,6 +102,7 @@ func writePrivKeyToFile(k crypto.PrivateKey, f *os.File, password string) error
 		return fmt.Errorf("unable to marshal the private key: %w", err)
 	}
 	bpemBlock := &pem.Block{
+		Type:  "PRIVATE KEY",
 		Bytes: b,
 	}
 	bpem := pem.EncodeToMemory(bpemBlock)
@@ -128,6 +129,7 @@ func writePubKeyToFile(k crypto.PublicKey, f *os.File) error {
 		return err
 	}
 	bpemBlock := &pem.Block{
+		Type:  "PUBLIC KEY",
 		Bytes: b,
 	}
 	bpem := pem.EncodeToMemory(bpemBlock)