Add caching for the Mill build tool

[clintval]

Description:

Similar to Gradle, Maven, and sbt, I am hoping to see Mill cache support.

Mill is becoming a popular JVM build tool so I hope this is a welcome addition.

I have not contributed to this project before so please let me know if I missed anything!

Speaking of, how do I run automated tests on this branch?

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull Request Overview

This PR adds caching support for the Mill build tool, expanding the action’s compatibility with popular JVM build tools.

• Adds Mill as a supported package manager with appropriate cache paths and file patterns in the source code.
• Introduces tests for Mill caching behaviors (error throwing, cache restoration consistency, etc.).
• Updates the documentation and end-to-end workflow to include examples and tests for Mill caching.

Reviewed Changes

Copilot reviewed 5 out of 9 changed files in this pull request and generated 1 comment.

File	Description
src/cache.ts	Adds Mill caching support to the PackageManager interface and supported array.
tests/cache.test.ts	Implements unit tests to verify Mill caching behavior.
README.md	Updates docs to include Mill as a supported package manager and usage examples.
.github/workflows/e2e-cache.yml	Adds e2e test workflow steps for saving/restoring Mill cache across OSes.

Files not reviewed (4)

• tests/cache/mill/.gitignore: Language not supported
• tests/cache/mill/.mill-version: Language not supported
• tests/cache/mill/build.sc: Language not supported
• tests/cache/mill/mill: Language not supported

[clintval]

I copied these patterns from above and notice a potential bug.

The matrix defines ubuntu-22.04 but the conditional here is still set to ubuntu-latest. I imagine this was not intentional. Is there a reason to pin to 22.04? If not, I'd vote reverting back to ubuntu-latest all over.

[lihaoyi]

_.compile is probably wrong (should be __.compile), but we should make this warmup command somethign the user passes in. Some users may want to cache jars, assemblies, or other artifacts beyond just plain compilation

[clintval]

Thanks for reviewing @lihaoyi!

_.compile should be OK here since this is just an integration for a tiny test Scala project:

setup-java/__tests__/cache/mill/build.sc

Lines 1 to 12 in 18d114c

	package build
	import mill._, scalalib._
	object MyProject extends ScalaModule {
	def scalaVersion = "2.13.11"
	def ivyDeps = Agg(ivy"com.lihaoyi::mainargs:0.6.2")
	object test extends ScalaTests {
	def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.8.5")
	def testFramework = "utest.runner.Framework"
	}
	}

I could change it to __.compile for safety-sake anyway, though.

And this isn't where the magic happens since these are just tests for this project.

The default caching behavior is set to:

setup-java/src/cache.ts

Lines 64 to 78 in 18d114c

	{
	id: 'mill',
	path: [
	join(os.homedir(), '.cache', 'mill')
	],
	pattern: [
	// https://github.com/coursier/cache-action/blob/4e2615869d13561d626ed48655e1a39e5b192b3c/README.md?plain=1#L28-L38
	'**/build.sc',
	'**/*.sc',
	'**/mill',
	'**/.mill-version',
	// https://github.com/com-lihaoyi/mill/blob/5b88d1e268e6264e44589c5ac82c0fdbd680fd63/mill#L6-L11
	'**/.config/mill-version'
	]
	}

So that anything under ~/.cache/mill is cached.

The GitHub Action cache is rebuilt if anything in these patterns changes:

pattern: [
  // https://github.com/coursier/cache-action/blob/4e2615869d13561d626ed48655e1a39e5b192b3c/README.md?plain=1#L28-L38
  '**/build.sc',
  '**/*.sc',
  '**/mill',
  '**/.mill-version',
  // https://github.com/com-lihaoyi/mill/blob/5b88d1e268e6264e44589c5ac82c0fdbd680fd63/mill#L6-L11
  '**/.config/mill-version'
]