Add a force-sdist input to the Python release workflow.

The first run for this month's release failed after uploading some
of the wheels, but before uploading the sdist, so we need a way to
force the sdist on a rerun if necessary.

Change-Id: I3851d20cdeba02caad5f450e77cd7ef8704990d2
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63170
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>

.github/workflows/python.yml

@@ -5,6 +5,10 @@ on:
       build:
         required: true
         type: number
+      force-sdist:
+        required: false
+        type: boolean
+        default: false
 permissions:
   contents: read
 jobs:
@@ -202,7 +206,7 @@ jobs:
           python -m pip install --upgrade pip
           python -m pip install --upgrade build wheel
         shell: bash
-      - if: inputs.build == 1
+      - if: inputs.build == 1 || inputs.force-sdist == true
         name: Build source
         env:
           SOURCE_DATE_EPOCH: ${{ env.timestamp }}