build: configure repository cache in .bazelrc (#4137)

Configure a shared Bazel repository cache and enable hardlinks for
local builds.

Developer workflows that use short-lived worktrees (such as agentic
development) repeatedly re-download external dependencies and consume
excessive disk space.

.bazelrc

diff --git a/.bazelrc b/.bazelrc
index cd41961..12d1b33 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -36,9 +36,6 @@
 # See https://github.com/bazel-contrib/rules_python/issues/3655
 common --incompatible_strict_action_env=false
 
-# To work around bug on bazel 7
-common:ci --experimental_repository_cache_hardlinks=false
-
 # Windows makes use of runfiles for some rules
 build --enable_runfiles
 
@@ -47,13 +44,17 @@
 
 # Local disk cache greatly speeds up builds if the regular cache is lost
 common --disk_cache=~/.cache/bazel/bazel-disk-cache
+# Shared repository cache avoids re-downloading external dependencies across workspaces
+common --repository_cache=~/.cache/bazel/bazel-repo-cache
+# Hardlinks save disk space and copy time when unpacking repositories
+common --experimental_repository_cache_hardlinks=true
+# To work around bug on bazel 7
+common:ci --experimental_repository_cache_hardlinks=false
 # Drop `experimental_` prefix once Bazel 7 is no longer supported
 common --experimental_downloader_config=downloader_config.cfg
 common --http_timeout_scaling=10.0
 common --experimental_repository_downloader_retries=10
 
-
-
 # Additional config to use for readthedocs builds.
 # See .readthedocs.yml for additional flags that can only be determined from
 # the runtime environment.