feat(cli.rs): expose debug flag to beforeDev/beforeBuild commands

[JonasKruckenberg]

Exposes the the --debug flag to beforeBuildCommand and beforeDevCommand through an environment variable called TAURI_DEBUG. This is useful for frontend bundlers to adjust their compilation based on the chosen build environment.
An example would be producing source maps for debug builds, but omitting them for release builds.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Docs
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: