tests: cache sample files

Author: henryborchers

.github/workflows/tox_matrix.yml

@@ -95,4 +95,4 @@ jobs:
         env:
           CONAN_COMPILER_LIBCXX: ${{ matrix.compiler_libcxx }}
           CONAN_USER_HOME: ${{ matrix.conan_user_home }}
-          SAMPLE_IMAGES_ARCHIVE: "{{ github.workspace }}/sample_images.tar.gz"
+          SAMPLE_IMAGES_ARCHIVE: "{{ github.workspace }}/sampledata/sample_images.tar.gz"