fix: overwrite existing environment variable

[msvticket]

appending an environment variable doesn't do anything if it is already set

The problem

If the BINARY_NAME environment variable happens to be set the help text are messed up:

$ BINARY_NAME=foo jx project --help
Create a new project by importing code, creating a quickstart or custom wizard for spring.

Usage:
  foo [flags]
  foo [command]

Examples:
  # Create a project using the wizard
  foo
---

As compared to the correct:

jx project --help
Create a new project by importing code, creating a quickstart or custom wizard for spring.

Usage:
  jx project [flags]
  jx project [command]

Examples:
  # Create a project using the wizard
  jx project

This PR fixes that:

BINARY_NAME=foo ./build/jx project --help
Create a new project by importing code, creating a quickstart or custom wizard for spring.

Usage:
  ./build/jx project [flags]
  ./build/jx project [command]

Examples:
  # Create a project using the wizard
  ./build/jx project
---

TOP_LEVEL_COMMAND has basically the same function as BINARY_NAME; some plugins use one and some the other.

[msvticket]

/cc tomhobson JordanGoasdoue Skisocks mentlak0

[msvticket]

/retest

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.11%. Comparing base (174064f) to head (0301b8c).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/cmd/root.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8898      +/-   ##
==========================================
+ Coverage   32.08%   32.11%   +0.02%     
==========================================
  Files          10       10              
  Lines        1175     1174       -1     
==========================================
  Hits          377      377              
+ Misses        755      754       -1     
  Partials       43       43              

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[jenkins-x-bot]

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