Change behavior of DEVTOOLS_TOOL_FLUTTER_FROM_PATH

[srawlins]

Previously, the DEVTOOLS_TOOL_FLUTTER_FROM_PATH variable was treated as a "is it present or not" thing. But that could be surprising, because then a value like DEVTOOLS_TOOL_FLUTTER_FROM_PATH=false would be considered... true!

This changes it to a value that must be true or false.

[gemini-code-assist]

Code Review

This pull request updates the tooling scripts and Dart command runner to check if the DEVTOOLS_TOOL_FLUTTER_FROM_PATH environment variable is explicitly set to "true" instead of just checking if it is defined or non-empty. The review feedback highlights that the bash scripts (tool/bin/dt and tool/ci/setup.sh) perform case-sensitive checks, which is inconsistent with the case-insensitive checks in the batch script and Dart runner, and suggests making them case-insensitive.