Use base browsers variable in Firestore karma config

[dlarocque]

Firestore's Karma config overwrites the browsers that are set in the base karma config:https://github.com/firebase/firebase-js-sdk/blob/main/packages/firestore/karma.conf.js#L24
getTestBrowsers(argv) only uses the BROWSERS environment variable if it's running the unit tests: https://github.com/firebase/firebase-js-sdk/blob/main/packages/firestore/karma.conf.js#L81
We want the BROWSERS environment to always define the browser to use- regardless of whether we're running unit tests.

[changeset-bot]

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Latest commit: b9b6aa9

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (dcc62c0) and merge commit (d4a5b87).

Test Logs

• Base (dcc62c0): https://github.com/firebase/firebase-js-sdk/actions/runs/14113941248
• Merge (d4a5b87): https://github.com/firebase/firebase-js-sdk/actions/runs/14115798121

1. https://storage.googleapis.com/firebase-sdk-metric-reports/kQPgIxMKKl.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (dcc62c0) and merge commit (d4a5b87).

Test Logs

• Base (dcc62c0): https://github.com/firebase/firebase-js-sdk/actions/runs/14113941248
• Merge (d4a5b87): https://github.com/firebase/firebase-js-sdk/actions/runs/14115798121

1. https://storage.googleapis.com/firebase-sdk-metric-reports/VvU4fSJaou.html