Since socket process can be launched before `GeckoDependentInitialize`, we cache the install directory in `InitSignedPolicyRulesToBypassCig` instead of using `sBinDir`. This patch moves `GetInstallDirectory` to WinHeaderOnlyUtils.h to reuse and removes `WinUtils::GetModuleFullPath` that is unused. Differential Revision: https://phabricator.services.mozilla.com/D133596