[wasm] Fix runtime tests build on CI

Runtime tests build for wasm started timing out recently, with the job
getting cancelled, likely due to the build hitting the container limits.

This changes the inner test builds to not run in parallel at all, with
`/m:1`, which unblocks the CI at least.

Issue: dotnet#93134

Author: radical

src/tests/build.proj

@@ -589,7 +589,9 @@
       <GroupBuildCmd>$(GroupBuildCmd) "/p:__TestGroupToBuild=$(__TestGroupToBuild)"</GroupBuildCmd>
       <GroupBuildCmd>$(GroupBuildCmd) "/p:__SkipRestorePackages=1"</GroupBuildCmd>
       <GroupBuildCmd>$(GroupBuildCmd) /nodeReuse:false</GroupBuildCmd>
-      <GroupBuildCmd>$(GroupBuildCmd) /maxcpucount</GroupBuildCmd>
+      <GroupBuildCmd Condition="'$(TargetOS)' != 'browser' or '$(ContinuousIntegrationBuild)' != 'true'">$(GroupBuildCmd) /maxcpucount</GroupBuildCmd>
+      <!-- https://github.com/dotnet/runtime/issues/93134 -->
+      <GroupBuildCmd Condition="'$(TargetOS)' == 'browser' and '$(ContinuousIntegrationBuild)' == 'true'">$(GroupBuildCmd) /m:1</GroupBuildCmd>
       <GroupBuildCmd Condition="'$(TargetOS)' == 'ios' or '$(TargetOS)' == 'tvos'">$(GroupBuildCmd) "/p:DevTeamProvisioning=-"</GroupBuildCmd>
       <GroupBuildCmd>$(GroupBuildCmd) /bl:$(ArtifactsDir)/log/$(Configuration)/InnerManagedTestBuild.$(__TestGroupToBuild).binlog</GroupBuildCmd>
       <GroupBuildCmd Condition="'$(CrossBuild)' == 'true'">$(GroupBuildCmd) "/p:CrossBuild=true"</GroupBuildCmd>