capture std err when getting required plugins for go

[EvanBoyle]

Fixes #4139

When go programs have compile time errors, we aren't capturing the error output. We obscure the error behind a generic "get required plugins" failure.

The new output for a program with a compile-time error looks like:

Evans-MBP:go_aws_qs evanboyle$ pulumi up
Previewing update (dev):
     Type                 Name           Plan     Info
     pulumi:pulumi:Stack  go_aws_qs-dev           2 messages
 
Diagnostics:
  pulumi:pulumi:Stack (go_aws_qs-dev):
    # go_aws_qs
    ./main.go:19:30: cannot use name (type interface {}) as type string in argument to s3.NewBucket: need type assertion
 
error: failed to discover plugin requirements: failed to execute program cmd: exit status 2

Note that we're already capturing stdout in line 172, so we don't redirect it here.

[lukehoban]

Do we intentionally not want to redirect SdOut as well?

[EvanBoyle]

from the description:

Note that we're already capturing stdout in line 172, so we don't redirect it here.

If you try to capture stdout, cmd.Output() will return an error saying that stdout is already bound.