Premature end of Content-Length delimited message body

Seen on Travis: note that it's fetching from the GCP Maven mirror.

```
=== RUN TestDebugEventsRPC_StatusCheck
time="2021-04-09T17:11:46Z" level=info msg="Running [skaffold build --default-repo gcr.io/k8s-skaffold] in testdata/jib"
helper.go:234: skaffold build: exit status 1, Generating tags...
- skaffold-jib -> gcr.io/k8s-skaffold/skaffold-jib:v1.21.0-35-g9045293
Checking cache...
- skaffold-jib: Error checking cache.
panic.go:617: getting hash for artifact "skaffold-jib": getting dependencies for "skaffold-jib": could not fetch dependencies for workspace .: initial Jib dependency refresh failed: failed to get Jib dependencies: running [/skaffold/integration/testdata/jib/mvnw jib:_skaffold-fail-if-jib-out-of-date -Djib.requiredVersion=1.4.0 --no-transfer-progress --non-recursive jib:_skaffold-files-v2 --quiet --batch-mode]
- stdout: "[ERROR] Failed to execute goal com.google.cloud.tools:jib-maven-plugin:3.0.0:_skaffold-fail-if-jib-out-of-date (default-cli) on project hello-java: Execution default-cli of goal com.google.cloud.tools:jib-maven-plugin:3.0.0:_skaffold-fail-if-jib-out-of-date failed: Plugin com.google.cloud.tools:jib-maven-plugin:3.0.0 or one of its dependencies could not be resolved: Could not transfer artifact org.apache.maven:maven-core:jar:3.6.3 from/to google-maven-central (https://maven-central.storage-download.googleapis.com/maven2/): GET request of: org/apache/maven/maven-core/3.6.3/maven-core-3.6.3.jar from google-maven-central failed: Premature end of Content-Length delimited message body (expected: 633,028; received: 309,904) -> [Help 1]\n[ERROR] \n[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.\n[ERROR] Re-run Maven using the -X switch to enable full debug logging.\n[ERROR] \n[ERROR] For more information about the errors and possible solutions, please read the following articles:\n[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginResolutionException\n"
- stderr: ""
- cause: exit status 1
```