[FIX] Don't allow SSH authentication without ssh executable

- Follow up of #4819
- When no `ssh` executable is present, disable the UI and backend bits
that allow the creation of push mirrors that use SSH authentication. As
this feature requires the usage of the `ssh` binary.
- Integration test added.

modules/git/git.go

@@ -38,6 +38,8 @@ var (
 	InvertedGitFlushEnv    bool // 2.43.1
 	SupportCheckAttrOnBare bool // >= 2.40
 
+	HasSSHExecutable bool
+
 	gitVersion *version.Version
 )
 
@@ -203,6 +205,10 @@ func InitFull(ctx context.Context) (err error) {
 		globalCommandArgs = append(globalCommandArgs, "-c", "filter.lfs.required=", "-c", "filter.lfs.smudge=", "-c", "filter.lfs.clean=")
 	}
 
+	// Detect the presence of the ssh executable in $PATH.
+	_, err = exec.LookPath("ssh")
+	HasSSHExecutable = err == nil
+
 	return syncGitConfig()
 }