Fix loading of core plugins on Windows

[2020-06-07T13:26:26.841+0300] [plugin-manager-preloader] ERROR com.googlesource.gerrit.plugins.manager.repository.CorePluginsRepository : Invalid plugin filename
java.net.URISyntaxException: Illegal character in opaque part at index 11: Jar:file:c:\Projects\gerrit\bin\gerrit.war!/WEB-INF/plugins/gitiles.jar
	at java.base/java.net.URI$Parser.fail(URI.java:2915)
	at java.base/java.net.URI$Parser.checkChars(URI.java:3086)
	at java.base/java.net.URI$Parser.parse(URI.java:3122)
	at java.base/java.net.URI.<init>(URI.java:600)
	at com.googlesource.gerrit.plugins.manager.repository.CorePluginsRepository.extractPluginInfoFromJarEntry(CorePluginsRepository.java:59)

Bug: Issue 12885
Change-Id: Icade084c2c156ca3caa1bffd9ae6dcde61a43c3e