msys2 build fixes (#59028) broke from source build

[benlorenz]

The scheduled build failed here
https://buildkite.com/julialang/julia-master-scheduled/builds/1218/steps/canvas?sid=019845bf-e676-4584-a8f5-19b0b047301f#019845bf-e6c0-4f91-b9e9-0865624e0210/872-889
with

[buildroot]/deps/llvm.mk:17: *** multiple target patterns.  Stop.

The same error happened in the Yggdrasil for julia 1.13 builds here:
https://buildkite.com/julialang/yggdrasil/builds/22212/steps/canvas?sid=0198447b-73a7-4f83-82dc-c341ef2ec24e#0198447b-743c-40ba-b938-6cf7e30a951f/692-1623

It seems that make doesn't like the colon in the rule expansion via

MSYS_NONEXISTENT_SYMLINK_TARGET_FIX := export MSYS=winsymlinks:native

and

$(SRCCACHE)/$(LLVM_SRC_DIR)/source-extracted: $(MSYS_NONEXISTENT_SYMLINK_TARGET_FIX)

Introduced in #59028
cc: @Keno @IanButterworth

[Keno]

I assume the export is probably syntax, so can't be expanded. I'll change it to only expand the setting.