Extract suggest_venv out of the else block

I think this is easier to read, but this is admittedly subjective.

This commit also makes the separate change of adjusting comment
spacing for consistency within the script. (Two spaces before "#"
is not widely regarded as better than one in shell scripting, so
unlike Python where PEP-8 recommends that, it would be equally good
to have changed all the other places in the shell scrips to have
just one.)

build-release.sh

@@ -9,18 +9,19 @@ function release_with() {
     "$1" -m build --sdist --wheel
 }
 
+function suggest_venv() {
+    local venv_cmd='python -m venv env && source env/bin/activate'
+    printf "HELP: To avoid this error, use a virtual-env with '%s' instead.\n" "$venv_cmd"
+}
+
 if test -n "${VIRTUAL_ENV:-}"; then
     deps=(build twine)  # Install twine along with build, as we need it later.
     echo "Virtual environment detected. Adding packages: ${deps[*]}"
     pip install --quiet --upgrade "${deps[@]}"
     echo 'Starting the build.'
     release_with python
 else
-    function suggest_venv() {
-        venv_cmd='python -m venv env && source env/bin/activate'
-        printf "HELP: To avoid this error, use a virtual-env with '%s' instead.\n" "$venv_cmd"
-    }
     trap suggest_venv ERR  # This keeps the original exit (error) code.
     echo 'Starting the build.'
-    release_with python3 # Outside a venv, use python3.
+    release_with python3  # Outside a venv, use python3.
 fi