Support NCNN_ISA environment variable detection

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Changes:

• Implemented get_isa_env function to retrieve and parse ISA flags from the environment variable.
• Added environment detection tests to CMake configuration, setting ISA env flags based on the system architecture.

Signed-off-by: Kaiyao Duan [email protected]

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Codecov Report

Attention: Patch coverage is 28.57143% with 45 lines in your changes missing coverage. Please review.

Project coverage is 95.82%. Comparing base (8998a13) to head (07a7892). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/cpu.cpp	28.57%	45 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5687      +/-   ##
==========================================
- Coverage   95.82%   95.82%   -0.01%     
==========================================
  Files         829      826       -3     
  Lines      270644   270140     -504     
==========================================
- Hits       259353   258867     -486     
+ Misses      11291    11273      -18     

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The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	16483752	16484400	+648 :warning:
armhf	7364300	7364516	+216 :warning:
aarch64	10712792	10779464	+66672 :warning: