Fix handling of file:// urls

Author: jkoritzinsky

Directory.Build.props

@@ -246,6 +246,8 @@
     <OfficialBaseURL>https://dotnetcli.azureedge.net/dotnet/</OfficialBaseURL>
     <!-- Allow overriding the public base URL for Unified Build scenarios to pull assets from a local build. -->
     <PublicBaseURL Condition="'$(PublicBaseURL)' == ''">https://dotnetbuilds.azureedge.net/public/</PublicBaseURL>
+    <!-- MSBuild removes the '//' slashes when passing PublicBaseURL from the outer to the inner build. -->
+    <PublicBaseURL Condition="$(PublicBaseURL.StartsWith('file:')) and '$(OS)' != 'Windows_NT'">$([System.Text.RegularExpressions.Regex]::Replace('$(PublicBaseURL)', '%28file:\/{1,}%29%28.+%29', 'file:///%242'))</PublicBaseURL>
     <InternalBaseURL>https://dotnetbuilds.azureedge.net/internal/</InternalBaseURL>
     <!-- Allow overriding where installers are pulled in from previously completed jobs in Unified Build scenarios. -->
     <CrossArchitectureInstallerBasePath Condition="'$(CrossArchitectureInstallerBasePath)' == ''">$(InstallersOutputPath)</CrossArchitectureInstallerBasePath>