If ${SDKROOT} is set - use it instead of calling xcrun

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Free-form text area

Please describe what your PR is adding in terms of features or bugfixes:

Compilation issues on macOS:

• After my mac upgraded to a new version of command-line tools for Xcode, the xcrun resolved to

xcrun --sdk macosx --show-sdk-path
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk

... which broke my compilation completely - see below.

• This seems to new for current conda-forge builds... The workaround is to set:

export SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX26.5.sdk/

This PR provides a workaround by allowing the devel tool allows to use an existing SDKROOT if already set.

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Compile log before the PR changes

ld: warning: ignoring file /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libSystem.tbd, malformed file
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libSystem.tbd:4:20: error: unknown architecture
                   arm64e.x1-macos, arm64e.x1-maccatalyst ]
                   ^~~~~~~~~~~~~~~

ld: warning: ignoring file /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libm.tbd, malformed file
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libm.tbd:4:20: error: unknown architecture
                   arm64e.x1-macos, arm64e.x1-maccatalyst ]
                   ^~~~~~~~~~~~~~~

Undefined symbols for architecture arm64:
  "___error", referenced from:
      _main in cc-370876.o
      ....```

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Declaration of use of AI-tools

• Please add a checkmark here if you used AI-tools during the work for this contribution
• Furter, please describe how / where and for what the tools were used:

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Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:

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PR Checklist for contributing to McStas/McXtrace

For a coherent and useful contribution to McStas/McXtrace, please fill in relevant parts of the checklist:

• My contribution contains something else

  • Explanation is added in free form text above or below the checklist

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[willend]

Related to #2657