MonoAOTCompiler: Check for proxy file's target only when the cache is…

[radical]

... being used.

Fixes #57800 .

[lewing]

hitting #57886 on
runtime (Build Android x64 Release AllSubsets_Mono_RuntimeTests) and
runtime (Build Browser wasm Release AllSubsets_Mono_RuntimeTests)

[lewing]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[lewing]

runtime (Libraries Test Run checked coreclr Linux_musl arm64 Release failed with:

./RunTests.sh: line 162:    93 Segmentation fault      (core dumped) "$RUNTIME_PATH/dotnet" exec --runtimeconfig System.Text.Json.Tests.runtimeconfig.json --depsfile System.Text.Json.Tests.deps.json xunit.console.dll System.Text.Json.Tests.dll -xml testResults.xml -nologo -nocolor -notrait category=IgnoreForCI -notrait category=OuterLoop -notrait category=failing $RSP_FILE
/root/helix/work/workitem
----- end Wed Aug 25 00:59:15 UTC 2021 ----- exit code 139 ----------------------------------------------------------
exit code 139 means SIGSEGV Illegal memory access. Deref invalid pointer, overrunning buffer, stack overflow etc. Core dumped.
ulimit -c value: unlimited

https://helixre8s23ayyeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-57865-merge-2f780863fc644dd9a1/System.Text.Json.Tests/1/console.97fdf128.log?sv=2019-07-07&se=2021-09-14T00%3A56%3A47Z&sr=c&sp=rl&sig=yb7KwfOQIWELVhob7I3S1eOUDHqYfj%2FSamRtp4rBA4A%3D

Looks like #57198

[lewing]

Failure is not related to the change.