SkipErrorOnPrebuilts no longer works

[corngood]

Since 5661a2c0a84a3fd32916395b254ce50e5ad7e9fe, I'm unable to build with --online without getting:

/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error : Prebuilt packages are not allowed in source-only builds.
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error :
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error : Detected 2 prebuilt package(s):
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error :   - runtime.linux-x64.Microsoft.NETCore.ILAsm.11.0.0-alpha.1.26064.118
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error :   - runtime.linux-x64.Microsoft.NETCore.ILDAsm.11.0.0-alpha.1.26064.118
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error :
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error : For detailed usage information, see: /var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/artifacts/log/Release/prebuilt-annotated-usage.xml
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error :
/var/tmp/nix-shell.TITB0b/dotnet-11.0.100-preview.1.26104.118/eng/finish-source-only.proj(138,5): error : See https://aka.ms/dotnet/prebuilts for guidance on what prebuilts are and how to eliminate them.

We do an online build in nixpkgs in order to fetch the required nuget dependencies for our bootstrap build. In the past SkipErrorOnPrebuilts was enough to make this work. Perhaps it should be taken into account on this new <Error> as well?

Originally posted by @corngood in dotnet/dotnet#3963 (comment)

[akoeplinger]

/cc @MichaelSimons

[corngood]

I'm currently using this in nixpkgs:

diff --git a/eng/finish-source-only.proj b/eng/finish-source-only.proj
index f1ff97926ea..b7d9f12d1e6 100644
--- a/eng/finish-source-only.proj
+++ b/eng/finish-source-only.proj
@@ -135,7 +135,7 @@
       <PrebuiltErrorMessage>$(PrebuiltErrorMessage)See https://aka.ms/dotnet/prebuilts for guidance on what prebuilts are and how to eliminate them.</PrebuiltErrorMessage>
     </PropertyGroup>
 
-    <Error Condition="'@(DetectedPrebuiltPackage)' != ''" Text="$(PrebuiltErrorMessage)" />
+    <Error Condition="'@(DetectedPrebuiltPackage)' != '' and '$(SkipErrorOnPrebuilts)' != 'true'" Text="$(PrebuiltErrorMessage)" />
 
     <MakeDir Directories="$(BaseIntermediateOutputPath)" />
     <Touch Files="$(BaseIntermediateOutputPath)ReportPrebuiltUsage.complete" AlwaysCreate="true">

I could create a PR, but I wasn't sure if this was the intended use of this flag.

[mthalman]

[Triage] @corngood - If you'd like to push your patch upstream, we'd be open to that. Currently we don't have a need for it but it looks like it would be useful for your scenario.

[corngood]

PR: dotnet/dotnet#5038