Fix failed docker CI using more reasoning grep regex to parse python version (aiidateam#6581)

The grep command failed that extracts the python version from mamba list failed because an indentation was added.
See the python string in the output of `mamba list`:

```
List of packages in environment: "/opt/conda"

  Name    Version  Build               Channel    
────────────────────────────────────────────────────
  python  3.10.13  hd12c33a_1_cpython  conda-forge
```

Author: unkcpz

.docker/aiida-core-base/Dockerfile

@@ -137,7 +137,7 @@ RUN set -x && \
         mamba && \
     rm micromamba && \
     # Pin major.minor version of python
-    mamba list python | grep '^python ' | tr -s ' ' | cut -d ' ' -f 1,2 >> "${CONDA_DIR}/conda-meta/pinned" && \
+    mamba list python | grep -oP 'python\s+\K[\d.]+' | tr -s ' ' | cut -d ' ' -f 1,2 >> "${CONDA_DIR}/conda-meta/pinned" && \
     mamba clean --all -f -y && \
     fix-permissions "${CONDA_DIR}" && \
     fix-permissions "/home/${SYSTEM_USER}"