Unbreak lint: drop vars context that the pinned actionlint rejects (#8490)

## Summary

ACTIONLINT has been failing on `main` since #8481, which introduced
`vars.PYTORCH_GREENLIGHT_MAX_DIFF_LINES` / `_BYTES` in
`greenlight-pr-review.yml`:

```
Error (ACTIONLINT) [expression]
  undefined variable "vars". available variables are "env", "github",
  "inputs", "job", "matrix", "needs", "runner", "secrets", "steps", "strategy"

  178 | MAX_DIFF_LINES: ${{ vars.PYTORCH_GREENLIGHT_MAX_DIFF_LINES || '2000' }}
  179 | MAX_DIFF_BYTES: ${{ vars.PYTORCH_GREENLIGHT_MAX_DIFF_BYTES || '500000' }}
```

`vars` is valid GitHub Actions syntax. The linter is simply stale:
`.lintrunner.toml` pins actionlint **1.6.21** via `s3_init_config.json`,
and the `vars` context was added in **1.6.24**.

## Why this blocks everything

The ACTIONLINT linter's `include_patterns` are `.github/workflows/*.yml`
/ `*.yaml`, so it lints **every** workflow regardless of what a PR
touched. Any PR therefore inherits this failure. Confirmed on `main`:

| run | commit | result |
|---|---|---|
| 31446317087 | `a6fa78f0b9` | failure |
| 31444723388 | `0e82c9f495` | failure |
| 31443941296 | `982575bb72` ← #8481 | failure |
| 31432664642 | `030feb596f` | success |

## The fix

Replaces the two lookups with literals carrying the same defaults, plus
a comment recording why and how to revert.

**Bumping actionlint is the better fix** — `vars` is legitimate and the
linter should understand it. I did not do that here because it needs new
binaries published to the `oss-clang-format` S3 bucket that
`s3_init_config.json` points at, which I can't write to. Worth doing as
a follow-up; this unblocks the repo in the meantime.

## Trade-off

The runtime override via repo variables is lost until the binary is
bumped — tuning the caps becomes a one-line edit rather than a repo
setting. Given the alternative is `main` staying red, that seemed the
right call, but say the word if you'd rather I wait for an actionlint
bump instead.

Behaviour is otherwise unchanged: the values were already the defaults,
and the step validates them as non-negative integers before use, so
nothing downstream is affected.

## Test plan

```
$ grep -rn 'vars\.' .github/workflows/ | grep -v 'env/vars.sh'
(only the explanatory comment remains)

$ node -e "yaml.load(...)"
YAML OK | jobs: [ announce_start, review, record ]
sizecheck env: {"MAX_DIFF_LINES":"2000","MAX_DIFF_BYTES":"500000"}
```

actionlint itself was not run locally — the binary is fetched by
`lintrunner init` from S3 — so CI is the real check. If `lintrunner`
goes green here, that confirms these two lines were the only remaining
ACTIONLINT failures.

cc @atalman

---

🤖 Authored with assistance from Claude Code.

Author: atalman

.github/workflows/greenlight-pr-review.yml

@@ -175,8 +175,15 @@ jobs:
       - name: Decline oversized diffs
         id: sizecheck
         env:
-          MAX_DIFF_LINES: ${{ vars.PYTORCH_GREENLIGHT_MAX_DIFF_LINES || '2000' }}
-          MAX_DIFF_BYTES: ${{ vars.PYTORCH_GREENLIGHT_MAX_DIFF_BYTES || '500000' }}
+          # Literals rather than `vars.*`: the actionlint pinned in .lintrunner.toml
+          # is 1.6.21, which predates the `vars` context (added in 1.6.24) and so
+          # reports it as an undefined variable. That failure is repo-wide, because
+          # the ACTIONLINT linter checks every workflow rather than only changed
+          # ones. Restore the `vars.PYTORCH_GREENLIGHT_MAX_DIFF_*` lookups once the
+          # pinned binary is bumped -- the step already rejects non-integer values,
+          # so it is safe against a malformed override.
+          MAX_DIFF_LINES: "2000"
+          MAX_DIFF_BYTES: "500000"
         run: |
           set -euo pipefail
           # Line count is the primary gate: the model reads only ~2000 lines of the diff, so a