popen: get correct cmd name on error

[gromgit]

Blindly using args[0] misleads users when an env is passed.

Before:

==> go build -ldflags=-s -w -X main.version=1.64.8 -X main.commit=8b37f14 -X main.date=2025-03-17T16:54:02Z ./cmd/golangci-lint
brew: command not found: {"SHELL"=>"bash"}
Error: Failure while executing; `\{\"SHELL\"=\>\"bash\"\} /opt/homebrew/Cellar/golangci-lint@1/1.64.8/bin/golangci-lint completion bash` exited with 127. Here's the output:

After:

==> go build -ldflags=-s -w -X main.version=1.64.8 -X main.commit=8b37f14 -X main.date=2025-03-17T16:54:02Z ./cmd/golangci-lint
brew: command not found: /opt/homebrew/Cellar/golangci-lint@1/1.64.8/bin/golangci-lint
Error: Failure while executing; `\{\"SHELL\"=\>\"bash\"\} /opt/homebrew/Cellar/golangci-lint@1/1.64.8/bin/golangci-lint completion bash` exited with 127. Here's the output:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

[MikeMcQuaid]

Thanks @gromgit!