Set TOOLCHAIN on BSD systems as well

[am11]

Both failed when building interop tests:

EXEC : error : unknown target triple 'unknown' [/runtime/src/libraries/System.Runtime.InteropServices/tests/TestAssets/NativeExports/NativeExports.csproj]
EXEC : error : unknown target triple 'unknown' [/runtime/src/libraries/System.Runtime.InteropServices/tests/TestAssets/NativeExports/NativeExports.csproj]
/root/.nuget/packages/dnne/2.1.2/build/DNNE.targets(259,5): error MSB3073: The command ""/usr/local/bin/clang-22" -g -O0 -shared -fpic -D DNNE_ASSEMBLY_NAME=Microsoft.Interop.Tests.NativeExports -D DNNE_COMPILE_AS_SOURCE -I "/root/.nuget/packages/dnne/2.1.2/tools/platform" -I "/runtime/artifacts/bin/openbsd-x64.Debug/corehost" -o "/runtime/artifacts/obj/NativeExports/Debug/net11.0/openbsd-x64/dnne/bin/Microsoft.Interop.Tests.NativeExportsNE.so" --target= --sysroot=/crossrootfs/x64  "/runtime/artifacts/obj/NativeExports/Debug/net11.0/openbsd-x64/dnne/Microsoft.Interop.Tests.NativeExports.g.c" "/root/.nuget/packages/dnne/2.1.2/tools/platform/platform.c" -lstdc++ "/runtime/artifacts/bin/openbsd-x64.Debug/corehost/libnethost.a" --target= --sysroot=/crossrootfs/x64 -fuse-ld=lld  " exited with code 1. [/runtime/src/libraries/System.Runtime.InteropServices/tests/TestAssets/NativeExports/NativeExports.csproj]
    0 Warning(s)
    3 Error(s)

On FreeBSD, this test project is disabled. I'm removing it downstream as bootstrap build for interop generator project succeeds after this.