Don’t set TEA_PREFIX

[mxcl]

Fixes pkgxdev/pkgx#427

We build to TEA_PREFIX by default in pantry dev-envs now. You can still override this if you want.

TEA_PANTRY_PATH=foo tea bar will now use

[what-the-diff]

• Update brewkit to 0.7
• Fix a bug in the build script where it was using {{ github.workspace }} instead of ${{ github.workspace }}. This is because we are now running this action inside an environment that uses mustache templates for variable substitution, so any variables must be escaped with $ if they should not be substituted by our template engine (which would break things).
• Add support for specifying package versions when building and testing packages via pkg specifiers like zlib^1.* or zlib~0.* etc... See https://github-redirector/teaxyz/pantry#pkg-specifier-syntax for more info on how these work)
• Remove some unnecessary files from .gitignore: tea_out, tea_linux - these were used as temporary directories during builds but have been replaced by srcs/, builds/, testbeds/. These new dirs will also get cleaned up automatically after each run of the workflow thanks to actions/cache@v2 which caches them between runs! :) Also remove deno lock file since we don't use deno anymore :P