ci: Fix Zizmor github-env failure in install_flutter action

[reidbaker]

Zizmor was flagging this failure in another review.
Agent suggested deleting this line since the following line does the same thing and I agree.
Secondary validation will be this pr passing presubmit.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• [] I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[stuartmorgan-g]

LGTM

[reidbaker]

Tree status is red but I believe it is a flake. The issue was a 500 style error downloading flutter.
Here is the rerun https://ci.chromium.org/ui/p/flutter/builders/prod/Linux_android%20android_platform_tests_shard_1%20stable/3488/overview