Use `pull_request_target` for coverage workflow. #2157

Job	Run time
firefox-container / DetailsTabUiTest	6m 38s
chrome-container / DetailsTabUiTest	6m 26s
firefox-container / FreeStyleConfigurationUiTest	2m 17s
chrome-container / FreeStyleConfigurationUiTest	2m 16s
firefox-container / GlobalConfigurationUiTest	2m 55s
chrome-container / GlobalConfigurationUiTest	3m 10s
firefox-container / IssuesColumnUiTest	3m 10s
chrome-container / IssuesColumnUiTest	3m 0s
firefox-container / SnippetGeneratorUiTest	3m 54s
chrome-container / SnippetGeneratorUiTest	3m 55s
firefox-container / TrendChartsUiTest	2m 45s
chrome-container / TrendChartsUiTest	2m 52s
firefox-container / WarningsPluginUiTest	5m 47s
chrome-container / WarningsPluginUiTest	5m 48s
firefox-container / SmokeTests	4m 7s
chrome-container / SmokeTests	4m 9s
	1h 3m 9s

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