chore: Replace devtools build_and_copy with a script that avoids the deprecated pwa-strategy flag

[spydon]

Running melos devtools-build fails with:

ERROR: ERROR: ERROR: The --pwa-strategy option is deprecated and will be removed in a future Flutter release.

The flag is not used anywhere in this repository. It is hardcoded inside the build_and_copy command of the devtools_extensions package, and every published version of that package still passes it, including the latest 0.5.1, so upgrading the dependency does not help.

This PR replaces the upstream command with scripts/devtools-build.sh, which mirrors it step by step without the deprecated flag: it runs the release web build of flame_devtools, fixes the canvaskit file permissions, and replaces the build folder under packages/flame/extension/devtools. The melos devtools-build command keeps its name, so the existing references in the READMEs remain valid.

Verified locally by running melos devtools-build, which now builds and copies the extension without the error.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• [-] I have updated/added tests for ALL new/updated/fixed functionality.
• [-] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

[spydon]

Wait for the fix upstream to land instead