perf: remove unnecessary calls to 'to_list'

[mattem]

Removes the unnecessary calls to to_list to retrieve the single file, when the attr is marked as allow_single_file, the file object can be obtained via ctx.file without the need to flatten the files depset.

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Type of change

• Bug fix (change which fixes an issue)
• Performance (a code change that improves performance)

For changes visible to end-users

• Suggested release notes are provided below:

Access toolchain bin via ctx.file to remove the need to flatten the files depset.

Test plan

• Covered by existing test cases

[alexeagle]

nice, how did you find it? Curious if there's something repeatable that belongs in a buildifier lint

[mattem]

Was using these as the basis for getting the toolchains working in rules_py and noticed it.