clear runtime layer on cache miss
clear runtime layer on cache miss

This commit fixes a bug where we failed to install a runtime after a cache miss which would cause issues when creating symlinks that already existed.

PiperOrigin-RevId: 445202607
Change-Id: I72ffa7c673d3645abfd54a90790dc9ef70916a90

GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks