Turn on --repository_cache in buildkite

Buildkite was re-extracting the sysroot for every single configuration.
This seems wasteful.

Change-Id: I981442f080892f84bf853e284ab3584266f1deca
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/tools/ci/buildkite.yaml b/tools/ci/buildkite.yaml
index 67a9a69..e5f5c4a 100644
--- a/tools/ci/buildkite.yaml
+++ b/tools/ci/buildkite.yaml
@@ -1,6 +1,6 @@
 env:
   STARTUP: --max_idle_secs=0
-  COMMON: -c opt --stamp=no --curses=yes --symlink_prefix=/ --remote_cache=grpc://data-fast:9092 --repo_env=FRC971_RUNNING_IN_CI=1
+  COMMON: -c opt --stamp=no --curses=yes --symlink_prefix=/ --remote_cache=grpc://data-fast:9092 --repo_env=FRC971_RUNNING_IN_CI=1 --repository_cache=~/.cache/bazel_repository --experimental_repository_cache_hardlinks=true
   TARGETS: //... @com_github_google_glog//... @com_google_ceres_solver//... @com_github_rawrtc_rawrtc//... @com_google_googletest//...
   M4F_TARGETS: //...
   RP2040_TARGETS: //...