Fix changeset checker

[hsubox76]

There is a bug in the latest changesets: changesets/changesets#517

Instead of letting it implicitly try to use master, use --since option to specify base SHA, obtained from Github Actions' env variables. Keep master/HEAD as a fallback option in case this script is run locally.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8b5552e

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[github-actions]

Changeset File Check ✅

• No modified packages are missing from the changeset file.
• No changeset formatting errors detected.

[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (61f0d7e) and head commit (47a0cec).

Test Logs

• Base (61f0d7ed): https://github.com/firebase/firebase-js-sdk/actions/runs/990971785
• Head (47a0cec3): https://github.com/firebase/firebase-js-sdk/actions/runs/991717315

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (61f0d7e) and head commit (47a0cec).

Test Logs

• Base (61f0d7e): https://github.com/firebase/firebase-js-sdk/actions/runs/990971785
• Head (47a0cec): https://github.com/firebase/firebase-js-sdk/actions/runs/991717315