Use a more optimized caching strategy for 'coverage-frontend'

Before change

Reference job: https://gitlab.com/gitlab-org/gitlab/-/jobs/552389308

Section	Duration (seconds)	Cumulative duration (seconds)	% of total	Can be improved at the job level?
Preparing the "docker+machine" executor	42	42	9.5 %	No
Preparing environment	4	46	0.9 %	No
Getting source from Git repository	89	135	20.3 %	No
Restoring cache	124	259	28.3 %	Yes
Downloading artifacts	8	267	1.8 %	Yes
yarn install	20	-	-	Yes
yarn node scripts/frontend/merge_coverage_frontend.js	5	-	-	Maybe
Running before_script and script	46	313	10.5 %	Yes
Running after_script	2	315	0.4 %	Yes
Saving cache	113	428	25.7%	Yes
Uploading artifacts for failed job	10	438	2.2  %	Yes

coverage-frontend duration is 7 minutes 18 seconds.

After change

Reference job: https://gitlab.com/gitlab-org/gitlab/-/jobs/552755732

Section	Duration (seconds)	Cumulative duration (seconds)	% of total	Can be improved at the job level?
Preparing the "docker+machine" executor	10 (-32)	10	6.7 %	No
Preparing environment	1 (-3)	11	0.6 %	No
Getting source from Git repository	59 (-30)	70	39.8 %	No
Restoring cache	50 (-74)	120	33.7 %	Yes
Downloading artifacts	7 (-1)	127	4.7 %	Yes
yarn install	1 (-19)	-	-	Yes
yarn node scripts/frontend/merge_coverage_frontend.js	4 (-1)	-	-	Maybe
Running before_script and script	7 (-39)	134	4.7 %	Yes
Running after_script	2 (-0)	136	1.3 %	Yes
Saving cache	2 (-111)	138	1.3%	Yes
Uploading artifacts for failed job	10 (-0)	148	2.2  %	Yes

coverage-frontend duration is 2 minutes 28 seconds (66% faster). 🚀

Closes #217751 (closed)