travis: Attempt to debug sccache failures

I can't find anything that'd cause unexpected EOF in the source, so let's try
taking a look at the error logs on failures.

.travis.yml

@@ -128,6 +128,7 @@ after_failure:
       echo "#### Build failed; Disk usage after running script:";
       df -h;
       du . | sort -nr | head -n100
+  - cat obj/tmp/sccache.log
 
 # Save tagged docker images we created and load them if they're available
 before_cache:

src/ci/docker/run.sh

@@ -30,13 +30,17 @@ retry docker \
 objdir=$root_dir/obj
 
 mkdir -p $HOME/.cargo
-mkdir -p $objdir
+mkdir -p $objdir/tmp
 
 args=
 if [ "$SCCACHE_BUCKET" != "" ]; then
     args="$args --env SCCACHE_BUCKET=$SCCACHE_BUCKET"
     args="$args --env AWS_ACCESS_KEY_ID=$AWS_ACCESS_KEY_ID"
     args="$args --env AWS_SECRET_ACCESS_KEY=$AWS_SECRET_ACCESS_KEY"
+    args="$args --env SCCACHE_ERROR_LOG=/tmp/sccache/sccache.log"
+    args="$args --env SCCACHE_LOG_LEVEL=debug"
+    args="$args --env RUST_LOG=sccache=debug"
+    args="$args --volume $objdir/tmp:/tmp/sccache"
 else
     mkdir -p $HOME/.cache/sccache
     args="$args --env SCCACHE_DIR=/sccache --volume $HOME/.cache/sccache:/sccache"