fix: scripts use project root as current working directory

[ericswanson-dfinity]

Description

Fixes a few places where dfx would execute commands in a subdirectory of a project, rather than setting the current working directory to the same directory as dfx.json.

This applies to:

• canister build scripts
• packtool command (vessel, mops and so forth)
• extension run
• tech stack value computation

Canister post-install scripts already set the working directory. This PR also adds a test that verifies this.

Fixes https://dfinity.atlassian.net/browse/SDK-1814

How Has This Been Tested?

Added e2e tests.

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [ ] I have made corresponding changes to the documentation.