Expand $SDK variable in mount plug targets

[jonathan-conder]

Description

I already started allowing this in SDKcraft, mostly to fix a mount slot validation bug quickly. But it looks like upcoming SDKs are going to want mount points in the SDK directory, so it makes sense to add $SDK for mount plugs in Workshop instead of removing it from SDKcraft.

Self-review quick check

• Make decisions that cost a lot to reverse explicit in the PR description.
• Avoid nested conditions.
• Delete dead code and redundant comments.
• Normalise symmetries by sticking to doing identical things identically.

// one way to handle errors
if err := f(); err != nil {
   ...
}

// one way to handle multiple returns
val, err := f()
if err != nil {
   ...
}
...

• Check that coupled code elements, files, and directories are adjacent. For example, test data is stored as close as possible to a test.
• Put variable declaration and initialisation together.
• Divide large expressions into digestable and self-explanatory ones. Use multiple variables if required.
• Put a blank line between two logically different chunks of code.
• Follow the style guide for new error messages.

Docs

• I have checked and added or updated relevant documentation.
• I have checked and added or updated relevant release notes.
• I have included the technical author in the review.

Or:

• I confirm the PR has no implications for documentation.

[akcano]

Minor improvements.

[github-actions]

TICS Quality Gate

✔️ Passed

workshop

All conditions passed

See the results in the TICS Viewer

The following files have been checked for this project

• internal/interfaces/builtin/mount.go
• internal/interfaces/builtin/tunnel.go

Automatic-tests / code-coverage / TICS GitHub Action