Custom GitHub action "restore-node" can fail on job retry

[gibson042]

Describe the bug

Our custom restore-node GitHub action captures the relevant endo branch in local file endo-sha.txt (which is also staged in git), then associates the agoric-sdk directory with an @actions/cache entry keyed in part upon that branch, then executes yarn install and yarn build only on cache miss, then finally does a "dirty check" that tolerates staged files (cf. #8168) but not untracked files. However, if the cache was populated by a job that removed its own endo-sha.txt (such as workflow integration "getting-started" with matrix.cli value "registry/yarn", "registry/npm", or "registry/npx", all of which include a "Start local NPM registry" step that executes git reset --hard HEAD), then the restored git directory will be unaware of the local file and report it as untracked, failing the action and its containing job.

This happened in the CI for #9688, where:

1. getting-started (registry/yarn) won the race to save the cache entry
   • Start local NPM registry
     1. git reset --hard HEAD
   • Post Run /./.github/actions/restore-node
     1. Cache saved with key: Linux-X64-22.04-node-18.18-built-0-c7d1c9f2923282881ba8d912fc314659e5e5a581-NOPE
2. getting-started (link-cli/yarn) failed due to what looks like flakiness (Possible race in integration testing #9710)
   • run agoric-cli integration-test
     1. [agd tx gov deposit] Error: rpc error: code = Unknown desc = rpc error: code = Unknown desc = account sequence mismatch, expected 63, got 62: incorrect account sequence [agoric-labs/[email protected]/x/auth/ante/sigverify.go:269] With gas wanted: '18446744073709551615' and gas used: '38588' : unknown request
3. a retry of getting-started (link-cli/yarn) failed in restore-node
   • Run /./.github/actions/restore-node
     1. Cache restored from key: Linux-X64-22.04-node-18.18-built-0-c7d1c9f2923282881ba8d912fc314659e5e5a581-NOPE
     2. …
     3. '[' -n '?? endo-sha.txt' ']' (from if [ -n "$(git status --porcelain | grep -Eve '^[^ ] '; true)" ]; then … exit 1)
     4. …
     5. Untracked files: … endo-sha.txt
     6. …
     7. Unexpected dirty git status

Expected behavior

restore-node should not expect that the endo-sha.txt file it creates is guaranteed to be included in a cache entry created at the end of its containing job. Specifically, I think it should tolerate absence of the file in its "dirty check" step.