Ensure libboringssl.a is always built for Python

[lidizheng]

Issue #17002

After discussing with @nicolasnoble , it is important to keep the BoringSSL inside the gRPC Python distribution. It is much easier to link inside the Python distribution than linking to the system's library, and may potentially create problems in Windows. So, instead of risking to build upon arbitrary dependencies, it seems more sensible to ensure libboringssl.a is always built for gRPC Python.

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 2,015,303      Total (=)      2,015,303

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
11,115,479      Total (<)     11,115,482

 No significant differences in binary sizes

[nicolasnoble]

Seems like macos aftifacts are failing :-)

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 2,015,303      Total (=)      2,015,303

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
11,115,999      Total (>)     11,115,995

 No significant differences in binary sizes

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 2,015,303      Total (=)      2,015,303

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
11,115,993      Total (<)     11,115,994

 No significant differences in binary sizes

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 2,015,303      Total (=)      2,015,303

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
11,115,993      Total (>)     11,115,986

 No significant differences in binary sizes

[Mottl]

Go is not installed (Artifact Build MacOS (internal CI)):

CMake Error at third_party/boringssl/CMakeLists.txt:38 (message):
304
  Could not find Go

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 2,015,649      Total (=)      2,015,649

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
11,116,198      Total (>)     11,116,193

 No significant differences in binary sizes

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 2,015,649      Total (=)      2,015,649

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
11,116,196      Total (>)     11,116,189

 No significant differences in binary sizes

[lidizheng]

Interestingly, this trunk of code specified for macOS was introduced by #10427.