Simplify envvar merging

.github/workflows/test.yml

@@ -17,13 +17,13 @@ defaults:
 
 env:
    # Note, that there's "turtle" as well, which is always excluded from running.
-   DEFAULT_PYTEST_SELECTION: "integration or usecase or slow or network"
-   DEFAULT_PYTEST_SELECTION_OP: "not "  # so it would be "not (integration or usecase)"
-   DEFAULT_DATALAD_TESTS_SSH: "1"
-   DEFAULT_DATALAD_LOG_ENV: GIT_SSH_COMMAND
+   PYTEST_SELECTION: "integration or usecase or slow or network"
+   PYTEST_SELECTION_OP: "not "  # so it would be "not (integration or usecase)"
+   DATALAD_TESTS_SSH: "1"
+   DATALAD_LOG_ENV: GIT_SSH_COMMAND
    # How/which git-annex we install.  conda's build would be the fastest,
    # but it must not get ahead in PATH to not shadow travis' python
-   DEFAULT_ANNEX_INSTALL_SCENARIO: "miniconda=py37_23.1.0-1 --python-match minor --batch git-annex=8.20201007 -m conda"
+   _DL_ANNEX_INSTALL_SCENARIO: "miniconda=py37_23.1.0-1 --python-match minor --batch git-annex=8.20201007 -m conda"
    BOTO_CONFIG: /tmp/nowhere
    DATALAD_DATASETS_TOPURL: https://datasets-tests.datalad.org
 
@@ -127,7 +127,7 @@ jobs:
       - name: Install git-annex
         run: |
           pip install datalad-installer
-          eval datalad-installer --sudo ok -E new.env ${_DL_ANNEX_INSTALL_SCENARIO:-$DEFAULT_ANNEX_INSTALL_SCENARIO}
+          eval datalad-installer --sudo ok -E new.env ${_DL_ANNEX_INSTALL_SCENARIO}
           # Append new.env to ~/.bashrc instead of $GITHUB_OUTPUT because it
           # can include `source` and `conda activate` commands that are invalid
           # for the latter; this also necessitates using an interactive shell
@@ -176,11 +176,6 @@ jobs:
 
       - name: Run tests
         run: |
-          : "${PYTEST_SELECTION:=$DEFAULT_PYTEST_SELECTION}"
-          : "${PYTEST_SELECTION_OP:=$DEFAULT_PYTEST_SELECTION_OP}"
-          : "${DATALAD_TESTS_SSH:=$DEFAULT_DATALAD_TESTS_SSH}"
-          : "${DATALAD_LOG_ENV:=$DEFAULT_DATALAD_LOG_ENV}"
-
           PYTEST_OPTS=( -v )
           # If we requested to run only not slow (typically <10sec) tests, fail
           # if a test takes 3x more than that - it needs to get @slow or