PLATFORM_MTL-12127: Fix sl_setup_env.py and build.sh for Git Bash on Windows

[Cedric Migliorini (migligli)]

Issue Link:
https://jira.silabs.com/browse/PLATFORM_MTL-12127

Description of Problem/Feature:
See this MD file OUT_PLATFORM_MTL-12127_Description-changes-to-scripts.md

Description of Fix/Solution:
See this MD file OUT_PLATFORM_MTL-12127_Description-changes-to-scripts.md

Testing Done:
I was able to run the following scripts on my Windows 11 OS machine from a git bash terminal:

python slc/sl_setup_env.py
./slc/build.sh slc/apps/lock-app/wifi/lock-app-917-ncp.slcw brd4186c

Details from the commit message:
sl_setup_env.py:

• Resolve SLT CLI from SILABS_SLT/SLT_EXE env, then PATH, before falling back to bundled slc/tools/slt. Skip download and slt update --self when using an external SLT.
• Add --use-bundled-slt flag to force the old behavior.
• Pass stdin=subprocess.DEVNULL to slt subprocess calls to prevent interactive prompts from hanging non-interactive runs.
• Single-quote all .env values on Windows so that semicolons in TOOLS_PATH, backslashes, and spaces (e.g. Simplicity Commander) are safe when build.sh sources the file in bash.

build.sh:

• Convert Windows TOOLS_PATH (semicolon-separated) to Unix PATH via cygpath -p -u when running under Git Bash / MSYS.
• Force OS=Windows_NT and pass OS, ARM_GCC_DIR, POST_BUILD_EXE, and USERPROFILE on the make command line so generated Makefiles select the correct Windows toolchain paths in recursive builds.
• Resolve POST_BUILD_EXE to a full path when .env provides only a basename (e.g. commander.exe).
• Accept slc.bat in addition to slc for the SLC CLI check.
• Fail early with a clear message when GNU Make is not on PATH.

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