Fix errcheck / gopls nits in main.go

[bradlarsen]

Description:

This PR fixes 3 nits identified by errcheck and the gopls LSP server:

• Adds missing error checking around a fallible os.SetEnv call
• Switches a loop over a call to strings.Split to call the more efficient strings.SplitSeq instead
• Switches an older-style counting loop to use the newer range-over-int added in Go 1.22 in Feb 2024

These are pedantic nits. The changes should not noticeably affect runtime behavior, but will quiet down editor tools.

Checklist:

• [X] Tests passing (make test-community)?
• [X] Lint passing (make lint this requires golangci-lint)?