[Bazel] Check cache directory exists before changing owners

[sjain-stanford]

This fixes a seeding issue with the previous PR where bazel build's GHA cache is not present to begin with and one of the commands (chown) fails on it. Should get the Bazel build back to green.

[sjain-stanford]

Co-incidentally, there's another failure after the rebase to develop (after this commit):

bazel-out/k8-fastbuild/bin/external/torch-mlir/_objs/TorchMLIRTorchPasses/Passes.pic.o:Passes.cpp:function
(anonymous namespace)::registerLowerToBackendContractPass()::{lambda()#1}::operator()()
const: error: undefined reference to 'mlir::torch::Torch::createLowerToBackendContractPass(int, bool)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Let me land this now, and send a follow-on PR to include "lib/Dialect/Torch/Transforms/LowerToBackendContract.cpp", to the TorchMLIRTorchPasses target.