.arcconfig

tests: use environment variable indirectly

Using environment variable directly in heredoc python code will cause
syntax error at checking module importation by import-checker.py
strictly, because "$varname" is invalid in Python syntax. "$varname"
becomes valid after environment variable substitution by shell at
writing text into file.

Current import-checker.py overlooks code fragment changed in this
patch, because of a restriction below for a line starting code
fragment.

  - filename must be specified before limit mark

    NG: cat <<EOF > FILE.py
    OK: cat > FILE.py <<EOF

import-checker.py itself is fixed in subsequent patch.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}