use correct (sequential) Mumps libraries in testing image

Conda forge recently updated their Mumps packaging so that the
sequential version of Mumps is in a separate package,
[mumps-seq](https://anaconda.org/conda-forge/mumps-seq),
as are the include files
[mumps-include](https://anaconda.org/conda-forge/mumps-include).

docker/kwant-latest.yml

@@ -11,7 +11,8 @@ dependencies:
     - qsymm
     - plotly
     # Linear algebra libraraies
-    - mumps
+    - mumps-seq
+    - mumps-include
     - blas #=1.1 openblas
     - openblas #=0.2.20
     # Build toolchain

docker/kwant-stable.yml

@@ -11,7 +11,8 @@ dependencies:
     - qsymm=1.2.6
     - plotly=2.2.2
     # Linear algebra libraraies
-    - mumps
+    - mumps-seq
+    - mumps-include
     - blas #=1.1 openblas
     - openblas #=0.2.20
     # Build toolchain