Fix undefined basis variable in interface workflow (#7571)

The "Create mock data & patch script" step referenced `basis` inside
the Python heredoc without defining it: the matrix key `basis` was
never exported to the step environment, so the step failed with
"NameError: name 'basis' is not defined" before any mock data was
generated.

Pass matrix.basis through the BASIS environment variable and read it
in the heredoc, matching the existing SCRIPT/PREFIX pattern.

Author: abacus_fixer

.github/workflows/interface.yml

@@ -58,12 +58,14 @@ jobs:
         env:
           SCRIPT: ${{ matrix.script }}
           PREFIX: ${{ matrix.prefix }}
+          BASIS: ${{ matrix.basis }}
         run: |
           python3 << 'PYEOF'
           import os, re, textwrap
 
           script = os.environ["SCRIPT"]
           prefix = os.environ["PREFIX"]
+          basis = os.environ["BASIS"]
 
           # ── 1. Patch the example script ────────────────────────
           with open(script) as f: