Extend the process env with new envvars rather than wholesale replacement

[addyess]

Environmental variables from the runner should be extended rather than replaced in exec calls

BugFix for #496

Overview

actions/exec when called with env will replace all the environment variables as evidenced here. Instead, we should extend the existing processes env variables with our new settings.

Rationale

Helps address missing PATH when calling charmcraft and other snap based tools.

Workflow Changes

No high level changes

Checklist

• The contributing guide was applied
• The PR is tagged with appropriate label (urgent, trivial, complex)

[addyess]

@arturo-seijas and @weiiwang01 do you folks have thoughts on this one?

[arturo-seijas]

Thanks for the contribution :)

[addyess]

The integration tests failed, but seem unreleated to my changes i think

[arturo-seijas]

The integration tests failed, but seem unreleated to my changes i think

Rerunning

[addyess]

ok, the publish job MIGHT be broken now... lemme look into that one

[addyess]

ok, the publish job MIGHT be broken now... lemme look into that one

I think i've determined this fails because I don't have access to the secret CHARMHUB_TOKEN in this repo. There seems to be no significant reason this shouldn't work