Fix #29: new input cabal-update: false to skip 'cabal update'

[andreasabel]

Fix #29: don't run 'cabal update'. New input cabal-update (default true) that can be set to false to turn off running cabal update.

This breaking behavior requires a major version bump (e.g. to v3.0.0).

[andreasabel]

I am not so sure about this change anymore. I think of this setup action as a "batteries included", maybe for the price of some redundancy.

[jneira]

what about a config switch to disable it?

[andreasabel]

what about a config switch to disable it?

Yes, that's an option, see #29 (comment).

[andreasabel]

CI: the "test project" seems to build even without cabal update: https://github.com/haskell/actions/actions/runs/3832554762/jobs/6522998203#step:5:5

Run cabal build
Warning: The package list for 'hackage.haskell.org' does not exist. Run 'cabal
update' to download it.

[jneira]

Thanks for making update optional, it would need change https://github.com/haskell/actions/blob/main/setup/README.md adding the new option

[andreasabel]

Thanks for making update optional, it would need change main/setup/README.md adding the new option

Ah, thanks for the alert!

[andreasabel]

I now added the description of cabal-update to the README.

This PR is ready again.

[andreasabel]

Planning to release this as 2.3.0. Minor version bump because of new feature.

[andreasabel]

Final description of this PR:

This commit introduces a new boolean input cabal-update that defaults to true.
If set to false, the cabal update step will be skipped.

The new flag cabal-update is proper boolean, it only accepts the YAML 1.2 truth values true and false (also in captial and uppercase).
This is contrast to other "boolean" flags that are "unset" by default and can be "set" with any non-empty string.

To test that cabal update happens, this commit also introduces a new Haskell test project into CI that depends on a common Hackage package (base-orphans).