viable strict: dedup two scripts, remove checkout of test-infra (#5750)

I discovered that you don't actually have to check out the test-infra
repository because it's already there?

Docs:
https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/store-information-in-variables#default-environment-variables

Only available for composite actions unfortunately

Testing:
https://github.com/pytorch/pytorch/actions/runs/11225822033?pr=137454
(actual pushing is commented out)

There are also two fetch_latest_green_commit.py scripts in test-infra so
I merged them and chose the location in tools/scripts. The logic between
the two is pretty much the same. gitutils was copied from
.github/scripts (but we don't need much of it)

There's also a reusable workflow update-viablestrict and a composite
action update-viablestrict. The composite action is usually used, but
there are some differences between the two, and vision uses the reusable
workflow so I can't get rid of it just yet. (idk how much they care tho,
I don't vision has succeeded updating viable/strict in a while). I also
can't just make the reusable action call the composite action since the
workflow uses an ssh key to check out but the action uses a token

---

If pytorch/vision#8676 gets merged, I'll remove
the reusable workflow

Author: clee2000

.github/actions/update-viablestrict/action.yml

@@ -55,13 +55,6 @@ runs:
       with:
         python-version: '3.11'
 
-    - name: Checkout test-infra for the fetch_latest_green_commit scripts
-      uses: actions/checkout@v3
-      with:
-        repository: ${{ inputs.test-infra-repository }}
-        ref: ${{ inputs.test-infra-ref }}
-        path: test-infra
-
     - uses: actions/checkout@v3
       with:
         repository: ${{ inputs.repository }}
@@ -85,8 +78,14 @@ runs:
       run: |
         set -ex
 
-        output=$(python ${GITHUB_WORKSPACE}/test-infra/.github/scripts/fetch_latest_green_commit.py --requires "${{ inputs.requires }}")
+        TEST_INFRA_PATH="${GITHUB_ACTION_PATH}/../../.."
+
+        output=$(python ${TEST_INFRA_PATH}/tools/scripts/fetch_latest_green_commit.py \
+          --required-checks "${{ inputs.requires }}" \
+          --viable-strict-branch "viable/strict" \
+          --main-branch "master")
         echo "latest_viable_sha=$output" >> "${GITHUB_OUTPUT}"
+        echo $output
 
     - name: Push SHA to viable/strict branch
       if: steps.get-latest-commit.outputs.latest_viable_sha != 'None'