Disable memory sanitizer due to lack of instrumented dependencies

[oskooi]

The clang MemorySanitizer run daily as part of the Github Actions workflow has recently been reporting this error for each of the 23 C++ unit tests in meep/tests:

SUMMARY: MemorySanitizer: use-of-uninitialized-value
/home/runner/work/meep/meep/src/time.cpp:34:41 in __cxx_global_var_init
Exiting
FAIL aniso_disp (exit status: 1)

This error is triggered by a global variable in time.cpp and is likely a false positive (since it has not previously appeared) as described in the LLVM docs:

MemorySanitizer requires that all program code is instrumented. 
This also includes any libraries that the program depends on, even libc. 
Failing to achieve this may result in false reports.

A possible fix is to compile libc++ with the instrumentation as described here.

For the time being, this PR disables the memory sanitizer.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.79%. Comparing base (f29a8c7) to head (74e20d4).
Report is 57 commits behind head on master.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3014      +/-   ##
==========================================
- Coverage   73.81%   73.79%   -0.03%     
==========================================
  Files          18       18              
  Lines        5423     5445      +22     
==========================================
+ Hits         4003     4018      +15     
- Misses       1420     1427       +7     

see 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.