scan-and-report.yml installs panic-attack from moving main — pin the scanner source (CWE-829)

[hyperpolymath]

.github/workflows/scan-and-report.yml installs the scanner with

cargo install --git https://github.com/hyperpolymath/panic-attack --branch main

A caller that pins the reusable workflow by SHA, for example echidna at @5ee2565…, still runs whatever binary main holds at run time. A push to main can therefore change every caller's scan results without changing any caller's pin (CWE-829). CodeRabbit raised this on hyperpolymath/echidna#399 (r4154702680). It is pre-existing: the earlier pin 27b3d93 has the same line.

Acceptance criteria

• The install step names an immutable revision (--rev <40-hex sha>), not --branch main.
• That revision is tied to the reusable's own revision, so bumping a caller's pin also moves the scanner. Either the workflow fixes --rev to the commit that contains it, or it takes an input defaulting to a SHA kept in step by the release process. The chosen mechanism is documented in the workflow header.
• A check, or a bash -n plus grep test, fails if --branch or a tag reappears in the install line.
• Callers (echidna, and any others found by gh search code "scan-and-report.yml@" and then verified by enumeration) are bumped to the fixed SHA.