♿ Add lang="en" to relevant snippets in build-system/

[caroqliu]

This PR is a partial copy of #31208, which adds lang="en" to relevant code snippets in this codebase. Instead of copying the PR file-for-file, I decided to break this change up into a few root directories for ease of review and less likelihood of falling behind to merge conflicts.

Original PR description:

Suggested changes and additions based on the TetraLogical accessibility review commissioned by @nainar / @caroqliu

x-ref ampproject/amp.dev#4972

/cc @TetraLogicalHelpdesk

[amp-owners-bot]

Hey @alanorozco! These files were changed:

build-system/server/app-index/amphtml-helpers.js

[kristoferbaxter]

These files are used by AMP developers working on the library itself?

[caroqliu]

These files are used by AMP developers working on the library itself?

Yes. In particular, the templates for the amp make-extension task are helpful to update as they create sample documents that people may use as exemplary. I don't feel strongly about changing the other two files. LMK if I should revert those.

[kristoferbaxter]

Changing them all makes sense, just wanted to make sure I understood where the value was.

Thanks!