Clean cache after conda segfault (#48)

Based on the logs in the run below, it seems that a retry alone is not
enough to resolve the segfault issues that have been appearing.

-
https://github.com/rapidsai/cudf/actions/runs/4298659489/jobs/7493440631#step:6:403

We should also try cleaning the cache based on the `invalid tarball`
messages just under the log line linked above.

tools/rapids-conda-retry

@@ -108,6 +108,7 @@ function runConda {
         elif [[ $exitcode -eq 139 ]]; then
             retryingMsg="Retrying, command resulted in a segfault. This may be an intermittent failure..."
             needToRetry=1
+            needToClean=1
         else
             rapids-echo-stderr "Exiting, no retryable ${RAPIDS_CONDA_EXE} errors detected: \
 'ChecksumMismatchError:', \