Remove restore-keys prefix fallback from GH-hosted build cache

The prefix fallback can restore a cache built on a runner with AVX-512
onto a runner without it, causing SIGILL in Chemistry tests. Without
restore-keys, only exact key matches are used — source changes trigger
a full rebuild but binaries are always compatible with the runner.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/coverage.yml

@@ -40,8 +40,6 @@ jobs:
         with:
           path: build
           key: mfc-coverage-${{ hashFiles('CMakeLists.txt', 'toolchain/dependencies/**', 'toolchain/cmake/**', 'src/**/*.fpp', 'src/**/*.f90') }}
-          restore-keys: |
-            mfc-coverage-
 
       - name: Setup Ubuntu
         run: |

.github/workflows/test.yml

@@ -99,8 +99,6 @@ jobs:
         with:
           path: build
           key: mfc-build-${{ matrix.os }}-${{ matrix.mpi }}-${{ matrix.debug }}-${{ matrix.precision }}-${{ matrix.intel }}-${{ hashFiles('CMakeLists.txt', 'toolchain/dependencies/**', 'toolchain/cmake/**', 'src/**/*.fpp', 'src/**/*.f90') }}
-          restore-keys: |
-            mfc-build-${{ matrix.os }}-${{ matrix.mpi }}-${{ matrix.debug }}-${{ matrix.precision }}-${{ matrix.intel }}-
 
       - name: Setup MacOS
         if:   matrix.os == 'macos'