fix error invoking kargo render

[krancour]

This is a follow-up to #1109.

I'm not sure how it was that I ever observed #1109 working correctly. All Kargo Render CLI invocations now fail.

The issue is that once cmd.Env for the child process exec'ing Kargo Render becomes non-nil (as it did starting in #1109), the child process no longer inherits its parent's environment -- the Go docs say so:

// Env specifies the environment of the process.
// Each entry is of the form "key=value".
// If Env is nil, the new process uses the current process's
// environment.
// If Env contains duplicate environment keys, only the last
// value in the slice for each duplicate key is used.
// As a special case on Windows, SYSTEMROOT is always added if
// missing and not explicitly set to the empty string.
Env []string

This causes PATH to be missing when the Kargo CLI spawns its own child processes to execute git commands.

This PR fixes the issue.

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[codecov]

Codecov Report

Attention: 25 lines in your changes are missing coverage. Please review.

Comparison is base (9007727) 50.37% compared to head (1ebba2e) 50.32%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/kubeclient/indexer.go	0.00%	23 Missing ⚠️
internal/controller/stages/stages.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1114      +/-   ##
==========================================
- Coverage   50.37%   50.32%   -0.05%     
==========================================
  Files         114      114              
  Lines        8076     8089      +13     
==========================================
+ Hits         4068     4071       +3     
- Misses       3861     3871      +10     
  Partials      147      147              

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