[mono] FullAOT runtime test failures: cross-compiler can't find sibling assemblies during AOT compilation #57371
Description
Breaks compilation of the assemblies in Loader/classloader/MethodImpl/CovariantReturns/UnitTest/UnitTestMultiModule.
Relevant environment variables used during AOT compilation:
2021-08-13T16:52:18.8495786Z Mono AOT options: full,llvm,llvm-path=/__w/1/s/artifacts/bin/mono/Linux.arm64.Release/cross/linux-arm64,tool-prefix=aarch64-linux-gnu-,mattr=crc,mattr=crypto
2021-08-13T16:52:18.8497092Z Mono AOT MONO_PATH: /__w/1/s/artifacts/tests/coreclr/Linux.arm64.Release/Tests/Core_Root
Example output from the arm64 LLVM FullAOT CI lane:
2021-08-13T17:38:19.0962792Z Mono Ahead of Time compiler - compiling assembly /__w/1/s/artifacts/tests/coreclr/Linux.arm64.Release/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/UnitTestMultiModule/B.dll
2021-08-13T17:38:19.0964289Z AOTID 30BFAE25-9FC8-BFB2-BECC-9B2CAA4B7E55
2021-08-13T17:38:19.0941328Z Failed to load method 0x6000001 from '/__w/1/s/artifacts/tests/coreclr/Linux.arm64.Release/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/UnitTestMultiModule/B.dll' due to Could not load file or assembly 'A, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies..
2021-08-13T17:38:19.0949223Z Run with MONO_LOG_LEVEL=debug for more information.
2021-08-13T17:38:19.0954757Z AOT of image /__w/1/s/artifacts/tests/coreclr/Linux.arm64.Release/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/UnitTestMultiModule/B.dll failed.
2021-08-13T17:38:19.1024852Z /__w/1/s/src/mono/msbuild/aot-compile.proj(8,9): error MSB3073: The command "/__w/1/s/artifacts/bin/mono/Linux.arm64.Release/cross/linux-arm64/mono-aot-cross /__w/1/s/artifacts/tests/coreclr/Linux.arm64.Release/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/UnitTestMultiModule/B.dll" exited with code 1.