Restore default key algorithm flag create source

This was removed by accident in the PR that introduced the new
`manifestgen` packages, and now restored in full glory.

Signed-off-by: Hidde Beydals <hello@hidde.co>

cmd/flux/create_source_git.go

@@ -122,6 +122,7 @@ func init() {
 
 func newSourceGitFlags() sourceGitFlags {
 	return sourceGitFlags{
+		keyAlgorithm:  flags.PublicKeyAlgorithm(sourcesecret.RSAPrivateKeyAlgorithm),
 		keyRSABits:    2048,
 		keyECDSACurve: flags.ECDSACurve{Curve: elliptic.P384()},
 	}