Fix security warnings from zizmor

[danielrbradley]

Experimenting with the new zizmor tool. There's still a number of false-positives so probably not yet worth integrating into our CI run, but have audited the current feedback.

Related to:

• https://github.com/pulumi/ci-mgmt/issues/1114

Only persist git credentials where we need to use them

• Don't leave these around when we don't need to.
• Explicitly set to true where we need them, with a comment highlighting why we're keeping them.
• Fix a few places we weren't using the centrally managed checkout version.
• Tweak the conditionals for submodules so the with: is always there now.

Use of fundamentally insecure workflow trigger - pull_request_target

These appear ok because we're just using this to comment on community PRs. These don't run builds

error[dangerous-triggers]: use of fundamentally insecure workflow trigger
  --> .github/workflows/community-moderation.yml:38:1
   |
38 | / on:
39 | |   pull_request_target:
...  |
42 | |     types:
43 | |     - opened
   | |_____________^ pull_request_target is almost always used insecurely
   |

error[dangerous-triggers]: use of fundamentally insecure workflow trigger
  --> .github/workflows/pull-request.yml:44:1
   |
44 | / on:
45 | |   pull_request_target: {}
   | |__________________________^ pull_request_target is almost always used insecurely
   |

Code injection via template expansion

.github/workflows/master.yml
  env.COVERAGE_OUTPUT_DIR may expand into attacker-controllable code

This is not inputtable by a third party user.

.github/workflows/prerequisites.yml
  inputs.default_branch may expand into attacker-controllable code

This is a workflow call (reusable workflow) and the input is always set as github.event.repository.default_branch.

.github/workflows/upgrade-provider.yml
  github.event.inputs.version may expand into attacker-controllable code
  steps.upstream_version.outputs.latest_version may expand into attacker-controllable code
  github.repository may expand into attacker-controllable code
  steps.target_version.outputs.version may expand into attacker-controllable code

This can only be triggered by internal users.

[t0yv0]

Thank you for doing this! Awesome.