ci: invoke pip via python -m in s3tests clone steps (#12010)

* ci: invoke pip via python -m in s3tests clone steps

The self-hosted runner reaches the Clone s3-tests step with the
setup-python toolcache interpreter on PATH but no pip shim, so the
bare 'pip install' exits 127. python -m pip resolves through the
interpreter that is already on PATH, matching the helm_ci workflow.

* ci: invoke pip via python3 -m in s3tests clone steps

The adjacent fix_s3_tests_bucket_conflicts step already runs python3, so
python3 is the interpreter guaranteed on the self-hosted PATH; a missing
python alias would fail the same way the missing pip shim did.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.github/workflows/s3tests.yml

@@ -58,9 +58,9 @@ jobs:
           cd s3-tests
           sudo apt-get update -qq
           sudo apt-get install -y -qq libxml2-dev libxslt1-dev zlib1g-dev
-          pip install -r requirements.txt
-          pip install tox
-          pip install -e .
+          python3 -m pip install -r requirements.txt
+          python3 -m pip install tox
+          python3 -m pip install -e .
 
       - name: Fix S3 tests bucket creation conflicts
         run: |
@@ -411,9 +411,9 @@ jobs:
           cd s3-tests
           sudo apt-get update -qq
           sudo apt-get install -y -qq libxml2-dev libxslt1-dev zlib1g-dev
-          pip install -r requirements.txt
-          pip install tox
-          pip install -e .
+          python3 -m pip install -r requirements.txt
+          python3 -m pip install tox
+          python3 -m pip install -e .
 
       - name: Fix S3 tests bucket creation conflicts
         run: |
@@ -582,9 +582,9 @@ jobs:
           cd s3-tests
           sudo apt-get update -qq
           sudo apt-get install -y -qq libxml2-dev libxslt1-dev zlib1g-dev
-          pip install -r requirements.txt
-          pip install tox
-          pip install -e .
+          python3 -m pip install -r requirements.txt
+          python3 -m pip install tox
+          python3 -m pip install -e .
 
       - name: Run S3 CORS tests
         timeout-minutes: 10
@@ -811,9 +811,9 @@ jobs:
           cd s3-tests
           sudo apt-get update -qq
           sudo apt-get install -y -qq libxml2-dev libxslt1-dev zlib1g-dev
-          pip install -r requirements.txt
-          pip install tox
-          pip install -e .
+          python3 -m pip install -r requirements.txt
+          python3 -m pip install tox
+          python3 -m pip install -e .
 
       - name: Run Ceph S3 tests with SQL store
         timeout-minutes: 15