[et] Fix path canonicalization

[cbracken]

Previously we were getting the enclosing directory path of the et tool (via the dirname builtin), then canonicalising the path by resolving symlinks. This doesn't work if the et on the path is itself a symlink. For example, if a user created a symlink et in a concrete directory at ~/.local/bin:

~/.local/bin/et -> ~/src/flutter/engines/src/flutter/bin/et

first we'd compute the dirname of the script:

~/.local/bin

then we'd resolve symlinks:

~/.local/bin

and incorrectly assume that was the engine directory.

Instead, we now resolve symlinks, then compute the enclosing directory.

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• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

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[flutter-dashboard]

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[zanderso]

You could test this change in the scenario you describe by adding a test to https://github.com/flutter/engine/blob/main/tools/engine_tool/test/entry_point_test.dart that creates such a symlink.