Add jsonnet tools to build image

[pracucci]

What this PR does / why we need it:
In #6254 I'm proposing to publish a compiled version of the mixin. To make it run in CI, we need to add some jsonnet tools to build image, which is what I'm doing in this PR. I've bumped the build image minor version (from 0.20 to 0.21) because I guess you follow semver.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
Changes have been tested in #6254

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[kavirajk]

LGTM 👍

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%