Add build folder to gitignore.

[lipzhu]

Default cmake build folder in vscode is "cmake.buildDirectory": "${workspaceFolder}/build".

[rjd15372]

For me it also makes sense to use the build directory for building valkey using cmake, which is the default directory for using cmake.

[madolson]

I feel like this came up at some point related to build-release. I'm OK with this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.85%. Comparing base (9f4503c) to head (f87c424).
Report is 26 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1488      +/-   ##
============================================
- Coverage     70.86%   70.85%   -0.02%     
============================================
  Files           119      119              
  Lines         64859    64860       +1     
============================================
- Hits          45963    45954       -9     
- Misses        18896    18906      +10     

see 14 files with indirect coverage changes