Attempt to fix CI

Give up on manually setting `LD_LIBRARY_PATH` as it doesn't seem to be
working, and instead use the `--prefix` flag to configure Poly/ML.

Author: hrutvik

.github/workflows/pure.yml

@@ -27,7 +27,6 @@ jobs:
     env:
       HOLDIR: ${{ github.workspace }}/HOL
       CAKEMLDIR: ${{ github.workspace }}/cakeml
-      LD_LIBRARY_PATH: /usr/local/lib:$LD_LIBRARY_PATH
 
     outputs:
       hol_sha: ${{ steps.shas.outputs.HOL_SHA }}
@@ -52,7 +51,7 @@ jobs:
     - name: Build Poly/ML
       run: |
         cd polyml
-        ./configure
+        ./configure --prefix=/usr
         make
         make install