try caching boost build

.github/workflows/ci.yml

@@ -52,6 +52,14 @@
 					uses: actions/checkout@v2,
 					with: { submodules: true }
 				},
+				{
+					name: Cache Boost,
+					uses: actions/cache@v2,
+					with: {
+						path: '${{ runner.workspace }}/boost_*.tar.gz',
+						key: 'boost-1.85.0'
+					}
+				},
 				{
 					name: Build Boost,
 					id: boost,