Fix test failures in local build due to environment variable filter

build_projects/dotnet-cli-build/EnvironmentVariableFilter.cs

@@ -28,6 +28,7 @@ namespace Microsoft.DotNet.Cli.Build
         private IEnumerable<string> _environmentVariablesToKeep = new string []
         {
             "DOTNET_CLI_TELEMETRY_SESSIONID",
+            "DOTNET_MULTILEVEL_LOOKUP",
             "DOTNET_RUNTIME_ID",
             "DOTNET_SKIP_FIRST_TIME_EXPERIENCE",
             "NUGET_PACKAGES"

run-build.sh

@@ -152,7 +152,7 @@ export VSTEST_TRACE_BUILD=1
 
 
 # Don't resolve shared frameworks from user or global locations
-DOTNET_MULTILEVEL_LOOKUP=0
+export DOTNET_MULTILEVEL_LOOKUP=0
 
 # Install a stage 0
 (set -x ; "$REPOROOT/scripts/obtain/dotnet-install.sh" --channel "release/2.0.0" --install-dir "$DOTNET_INSTALL_DIR" --architecture "$ARCHITECTURE" $LINUX_PORTABLE_INSTALL_ARGS)