CI: Track backend code coverage (#100856)

* CI: Track backend code coverage

This is a super rudimentary way to track this coverage. The important bit for me is the ability to extract the coverage
files.

* CI: Allow tests to fail

* Codeowners: Assign ownership of coverage tracking

* CI: Join coverage info in the job

* CI: Attempt to parallellise tests

* CI: Upload despite failures

* CI: Pattern is not regex

* CI: Set up repository and Go before merging

* CI: Generate go before checking coverage

* CI: Multi-line string

* CI: Backticks execute commands; avoid them

* CI: Make the output a bit prettier

Tabs are absurdly large.

* CI: Remove comment on retention

Makefile

@@ -248,8 +248,7 @@ test-go: test-go-unit test-go-integration
 .PHONY: test-go-unit
 test-go-unit: ## Run unit tests for backend with flags.
 	@echo "test backend unit tests"
-	printf '$(GO_TEST_FILES)' | xargs \
-	$(GO) test $(GO_RACE_FLAG) -short -covermode=atomic -timeout=30m
+	$(GO) test $(GO_RACE_FLAG) -short -covermode=atomic -coverprofile=unit.cov -timeout=30m $(GO_TEST_FILES)
 
 .PHONY: test-go-unit-pretty
 test-go-unit-pretty: check-tparse
@@ -262,7 +261,7 @@ test-go-unit-pretty: check-tparse
 .PHONY: test-go-integration
 test-go-integration: ## Run integration tests for backend with flags.
 	@echo "test backend integration tests"
-	$(GO) test $(GO_RACE_FLAG) -count=1 -run "^TestIntegration" -covermode=atomic -timeout=5m $(GO_INTEGRATION_TESTS)
+	$(GO) test $(GO_RACE_FLAG) -count=1 -run "^TestIntegration" -covermode=atomic -coverprofile=integration.cov -timeout=5m $(GO_INTEGRATION_TESTS)
 
 .PHONY: test-go-integration-alertmanager
 test-go-integration-alertmanager: ## Run integration tests for the remote alertmanager (config taken from the mimir_backend block).