add cached boost to github actions

.github/workflows/cache.yml

@@ -0,0 +1,20 @@
+- name: Cache Boost
+  uses: actions/cache@v2
+  with:
+    path: '${{ runner.workspace }}/boost_*.tar.gz'
+    key: 'boost-1.85.0'
+
+- name: Build Boost
+  # This won't re-download the archive unnecessarily:
+  uses: egor-tensin/build-boost@v1
+  with:
+    version: 1.85.0
+    libraries: align assert atomic config core predef preprocessor static_assert throw_exception type_traits
+    platform: x64
+    configuration: Release
+
+- name: Show paths
+  run: |
+    printf 'Boost has been unpacked to: %s\n' '${{ steps.boost.outputs.root }}'
+    printf 'Libraries can be found here: %s\n' '${{ steps.boost.outputs.librarydir }}'
+  shell: bash