Add symlink that points to current mono sdks dir

[rolfbjarne]

Use that instead of an msbuild variable that won't work in VSMac.

Also bump maccore to get an equivalent fix there.

Diff: https://github.com/xamarin/maccore/compare/34725fe136aee39a9f9984bbb508b6670654f7b3..104656b89060d3c98cdc626709f7cadc467549af

[rolfbjarne]

This is the part of #6097 that's required to make the tip of maccore to build (#6097 requires breaking changes in maccore which has already been implemented; this PR makes sure master is still buildable with the tip of maccore/master in case #6097 is delayed).

[monojenkins]

Build failure
✅ Build succeeded
✅ API Diff (from stable)
✅ API Diff (from PR only) (no change)
✅ Generator Diff (no change)
🔥 Test run failed 🔥

Test results

1 tests failed, 102 tests passed.

Failed tests

• dont link/Mac Classic/All: BuildFailure

[rolfbjarne]

build

[monojenkins]

Build failure
✅ Build succeeded
✅ API Diff (from stable)
✅ API Diff (from PR only) (no change)
✅ Generator Diff (no change)
🔥 Test run failed 🔥

Test results

3 tests failed, 166 tests passed.

Failed tests

• monotouch-test/watchOS 32-bits - simulator/Debug: Crashed
• [NUnit] Mono SystemJsonTests/watchOS 32-bits - simulator/Debug: Crashed
• [xUnit] Mono SystemNetHttpUnitTestsXunit/watchOS 32-bits - simulator/Debug: Crashed

[rolfbjarne]

Test failures on public Jenkins are unrelated:

• */watchOS: https://github.com/xamarin/maccore/issues/581

[spouliot]

known issues

• HE0038 -> https://github.com/xamarin/maccore/issues/581
• 3x tests app without test cases -> fixed in another PR
• certificate pinning on 10.10 -> https://github.com/xamarin/maccore/issues/1645