fix: Update DRONE branch used for Dashboard

[jennifer-shehane]

• close Should use DRONE_SOURCE_BRANCH #17860

User Facing Changelog

• Update the branch collected for drone CI to reflect the source PR branch.

Additional Details

We previously used the DRONE_COMMIT_BRANCH which is actually the target branch the PR is being merged into. What we want is the DRONE_SOURCE_BRANCH which is the actual branch of the PR.

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[cypress]

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Test summary

4184 • 2 • 53 • 1 • 3

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Run details

Project	cypress
Status	Failed
Commit	078e6bd
Started	Sep 14, 2021 3:04 PM
Ended	Sep 14, 2021 3:16 PM
Duration	12:19 💡
OS	Linux Debian - 10.9
Browser	Firefox 89

View run in Cypress Dashboard ➡️

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Failures

	commands/actions/type_spec.js		1
1	... > element
	e2e/focus_blur_spec.js		1
1	intercept blur methods correctly > focus area

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Flakiness

	net_stubbing_spec.ts		2
1	network stubbing > intercepting request > can intercept utf-8 request bodies without crashing
2	network stubbing > waiting and aliasing > can timeout incrementally waiting on requests
	xhr_spec.js		1
1	src/cy/commands/xhr > abort > aborts xhrs currently in flight

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[cypress-bot]

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