Replace hardcoded shebangs with /usr/bin/env for portability

[Lillecarl]

What this PR does / why we need it:

Replaces hardcoded shebang paths (#!/bin/bash and #!/bin/sh) with portable versions (#!/usr/bin/env bash and #!/usr/bin/env sh) in all shell scripts in the hack/ directory.

This improves portability across different Unix-like systems where shell interpreters may not be in standard locations, such as:

• NixOS: Bash is located at /run/current-system/sw/bin/bash
• BSD variants: May have bash in /usr/local/bin/bash
• Custom environments: Where bash is installed in non-standard locations

Using /usr/bin/env is the POSIX-recommended approach and is already widely adopted in the Go ecosystem.

Which issue(s) this PR fixes:

Fixes #1104

Does this PR introduce a user-facing change?

NONE

Additional Notes for your reviewer:

This is a straightforward portability fix that doesn't change any functionality. All 10 shell scripts in the hack/ directory have been updated. The scripts will continue to work exactly as before on systems with bash in /bin/bash, while now also working on systems where bash is in other locations.

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated (N/A - shebang-only change)
• Relevant docs in this repo added or updated (N/A - no doc changes needed)
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR (N/A - no doc changes needed)
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

N/A

[CodesbyUnnati]

LGTM, Thanks for raising this PR.