//tests/integration:UnsafeSharedCacheTest failed to build on Ubuntu

Since https://buildkite.com/bazel/rules-jvm-external/builds/1199, this test is failing to build with the following error message:
```
Opening zip "external/unsafe_shared_cache/v1/https/repo1.maven.org/maven2/com/google/guava/guava/27.0-jre/guava-27.0-jre.jar": open(): No such file or directory
Unable to open Zip file external/unsafe_shared_cache/v1/https/repo1.maven.org/maven2/com/google/guava/guava/27.0-jre/guava-27.0-jre.jar: No such file or directory
```

I did some investigation and found that it will only fail to build when remote cache is enabled.
Because it was green on the same commit before build 1199, the issue looks like caused by infra change.

Another point worth notice is `external/unsafe_shared_cache/v1/https/repo1.maven.org/maven2/com/google/guava/guava/27.0-jre/guava-27.0-jre.jar` is a symlink pointing to another location. The file at the destination does exist.

/cc @jin @philwo

Start with //tests/integration:UnsafeSharedCacheTest and reproduce the Ubuntu build with remote cache enabled, then compare it with remote caching disabled. Inspect the reported Guava JAR symlink and the Buildkite change around build 1199; done means the test builds successfully with remote caching enabled.

Written by the indexing model from the issue text.