[build-presets] Disable SourceKit-LSP tests for SwiftPM Linux CI jobs

SourceKit-LSP tests are flaky on Linux, disable them until we have fixed the root cause.

Resolves rdar://89359439

Author: ahoppen

utils/build-presets.ini

@@ -1705,6 +1705,8 @@ mixin-preset=mixin_swiftpm_linux_platform
 
 skip-test-llbuild
 skip-test-swiftpm
+# SourceKit-LSP tests are flaky on Linux, disable them until we have fixed the root cause - rdar://90437872
+skip-test-sourcekit-lsp
 
 
 #===------------------------------------------------------------------------===#