Attempt to use null separation when invoking 'env'

[cottsay]

This approach eliminates the heuristics for handling newlines in environment variables on platforms where 'env' supports null separation via the '-0' flag. The 'sh' shell will first attempt to use null separation and will fall back to the prior behavior.

The existing heuristics make some pretty critical assumptions about environment variable naming that can misbehave in extremely cryptic ways.

A specific example is that the regular expression used to determine if a line contains a valid environment variable name is based on the characters that Bourne shells accept when setting environment variables, but the set of acceptable characters in Linux is wider and the shells may still pass those variables through to subprocesses even though they don't support setting new variables with such names. The existing code will treat these variables as a continuation of the previous variable. If the previous variable is one that we depend on, such as AMENT_PREFIX_PATH, we can see cryptic problems during the build that don't appear to be related to colcon.

The heuristics were added in #128 to handle multi-line environment variables.

[codecov]

Codecov Report

Attention: Patch coverage is 73.07692% with 7 lines in your changes missing coverage. Please review.

Project coverage is 87.18%. Comparing base (a74fa97) to head (5fc80a7).

Files with missing lines	Patch %	Lines
colcon_core/shell/__init__.py	78.94%	4 Missing ⚠️
colcon_core/shell/sh.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
- Coverage   87.28%   87.18%   -0.10%     
==========================================
  Files          68       68              
  Lines        3949     3973      +24     
  Branches      760      763       +3     
==========================================
+ Hits         3447     3464      +17     
- Misses        396      403       +7     
  Partials      106      106              

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[christophebedard]

Just to mention it here:

I tested this and it fixes ros2/rosidl_typesupport#162, see ros2/rosidl_typesupport#162 (comment)