BUG: rm quotes (') from around env-var values

because GitHub Actions inserts the quotes in
the values of the environment variables that
are created from the assignments that are
printed by the script `setup_shell_env.py`.

For example, the line in the file `release.yml`:

```yaml
asset_path: tools/installer/${{ env.INSTALLER }}
```

results, when interpreted by GitHub Actions,
in the value:

```
asset_path: tools/installer/'tlaps-1.5.0-x86_64-linux-gnu-inst.bin'
```

which raises in CI the error:

```
Error: ENOENT: no such file or directory, stat 'tools/installer/'tlaps-1.5.0-x86_64-linux-gnu-inst.bin''
```

.github/workflows/setup_shell_env.py

@@ -38,10 +38,21 @@ def _make_shell_definitions(kv: dict) -> str:
     `kv=dict(NAME='value')`,
     returns the string`"NAME='value'"`.
     """
+    any_blanks = any(map(
+        _contains_blanks,
+        kv.values()))
+    if any_blanks:
+        raise ValueError(kv)
     return '\n'.join(
-        f"{name}='{value}'"
+        f'{name}={value}'
         for name, value in kv.items())
 
 
+def _contains_blanks(value: str) -> bool:
+    """Return `True` if blankspace in `value`."""
+    value_ = ''.join(value.split())
+    return len(value) != len(value_)
+
+
 if __name__ == '__main__':
     _main()