run_tests: leverage cipd when available for help2man

This tool isn't installed on CI bot images so we've been skipping it,
but this is causing people to not run tests locally, and ignore errors.
Use cipd to pull the tool in when available.

Then revert a recent man change that the tool rejects.

Change-Id: I1030d0070fd5a624656eba7434ae6ec99b2e3f2d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/582401
Reviewed-by: Greg Edelston <gredelston@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

man/repo-manifest.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "May 2026" "repo manifest" "Repo Manual"
+.TH REPO "1" "April 2026" "repo manifest" "Repo Manual"
 .SH NAME
 repo \- repo manifest - manual page for repo manifest
 .SH SYNOPSIS
@@ -697,7 +697,7 @@ default to which all projects in the included manifest belong. This recurses,
 meaning it will apply to all projects in all manifests included as a result of
 this element.
 .PP
-Local Manifests 
+Local Manifests
 .PP
 Additional remotes and projects may be added through local manifest files stored
 in `$TOP_DIR/.repo/local_manifests/*.xml`.