Avoid possible hang in util.RunCmdOut by using byte buffers instead of pipes

[briandealwis]

Our util.RunCmdOut() function runs commands with creating pipes for stdout and stderr, and then reads each of them to completion:

skaffold/pkg/skaffold/util/cmd.go

Lines 78 to 95 in 95092ad

	stderrPipe, err := cmd.StderrPipe()
	if err != nil {
	return nil, err
	}
	if err := cmd.Start(); err != nil {
	return nil, fmt.Errorf("starting command %v: %w", cmd, err)
	}
	stdout, err := ioutil.ReadAll(stdoutPipe)
	if err != nil {
	return nil, err
	}
	stderr, err := ioutil.ReadAll(stderrPipe)
	if err != nil {
	return nil, err
	}

But this code may hang if the stderr pipe fills and cause the process to block.

Since this code does not actually monitor the output streams, this PR changes the implementation to use byte buffers instead and let the Go runtime manage reading and writing.

[codecov]

Codecov Report

Merging #5220 (bb7e12f) into master (c62d6f5) will increase coverage by 0.04%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master    #5220      +/-   ##
==========================================
+ Coverage   71.86%   71.90%   +0.04%     
==========================================
  Files         388      388              
  Lines       14051    14043       -8     
==========================================
  Hits        10098    10098              
+ Misses       3207     3203       -4     
+ Partials      746      742       -4     

Impacted Files	Coverage Δ
pkg/skaffold/util/cmd.go	66.66% <81.81%> (+15.23%)	⬆️

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