govc: Fix memory aliasing in for loop (gosec G601)

[atc0005]

Description

Explicitly reassign the iteration variable used for listing virtual machine snapshots to fix the gosec G601 "Implicit memory aliasing in for loop" error.

Type of change

Please mark options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update
• Build related change

How Has This Been Tested?

• go build ./...
• go test -v ./govc

Checklist:

• My code follows the CONTRIBUTION guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

[dougm]

Thanks @atc0005