add MKL lib dir to LD_LIBRARY_PATH in GH actions

Since Nutils doesn't search for MKL installation directories any longer, we
need to tell GitHub actions where to find MKL manually. This patch adds a
search script to the devtools and updates `LD_LIBRARY_PATH` before running the
unittests.

.github/workflows/test.yaml

@@ -97,7 +97,13 @@ jobs:
           # Install Nutils from `dist` dir created in job
           # `build-python-package`.
           python -um pip install --no-index --find-links ./dist nutils
+      - name: Get library directory
+        id: get-lib-dir
+        if: ${{ matrix.matrix-backend == 'mkl' }}
+        run: python -um devtools.gha.get_lib_dir
       - name: Test
+        env:
+          LD_LIBRARY_PATH: ${{ steps.get-lib-dir.outputs.libdir }}
         run: |
           mkdir testenv
           cp -r examples docs tests .coveragerc testenv

devtools/gha/get_lib_dir.py

@@ -0,0 +1,16 @@
+from .. import log
+import sys, site, os
+
+libname = 'libmkl_rt.so'
+libsubdir = 'lib'
+prefixes = list(site.PREFIXES)
+if hasattr(site, 'getuserbase'):
+  prefixes.append(site.getuserbase())
+for prefix in prefixes:
+  libdir = os.path.join(prefix, libsubdir)
+  if not os.path.exists(os.path.join(libdir, libname)):
+    continue
+  log.set_output('libdir', libdir)
+  break
+else:
+  log.error('cannot find {} in any of the following dirs: {}'.format(libname, ' '.join(prefixes)))