Workflow cleanup: move checkout to start, prune unused steps+vars.

[ScottTodd]

Cutting down on boilerplate and standardizing some workflow syntax before deeper refactoring work.

• Move users: actions/checkout to the start of each job (for consistency, since all workflows use the source code but not all workflows need dependencies or python setup, etc.)
• Per https://github.com/actions/checkout, submodules: false is the default
• Step names can be omitted with uses (docs: https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#jobsjob_iduses) . Our pre-commit.yml workflow does this already:

  shark-ai/.github/workflows/pre-commit.yaml

  Lines 11 to 14 in c0ca2e2

  	steps:
  	- uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
  	- uses: actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b # v5.3.0
  	- uses: pre-commit/action@2c7b3805fd2a0fd8c1884dcaebf91fc102a13ecd # v3.0.1

  
  
• Remove copy/pasted Get Current Date step from some workflows
• Remove unused SHARK_PLATFORM_REPO_ROOT env var from some workflows (unused since [sharktank] Evaluation - Update timeout for Perplexity CI test #305, copy/pasted into a few places)

[stbaione]

LGTM, test failures for perplexity short and Llama Benchmarking are due to #638, not anything with this PR

[ScottTodd]

Ah. Thanks for noticing. Sent #642

[marbre]

Thanks for the cleanup.