hack scripts doesn't respect $PATH #1104

What steps did you take:

Attempted to run kapp's build and test scripts (located in hack/ directory) on a system where bash is not located at /bin/bash (e.g., NixOS, some BSD variants, or custom configurations).

What happened:

Scripts fail to execute because they use hardcoded shebang paths like #!/bin/bash and #!/bin/sh, which assume the shell interpreter is in a specific location.

What did you expect:

Scripts should be portable and work across different Unix-like systems regardless of where the shell interpreter is installed. Using #!/usr/bin/env bash and #!/usr/bin/env sh allows the system to locate the interpreter via the PATH environment variable.

Anything else you would like to add:

This affects all shell scripts in the hack/ directory:

• hack/build-and-lint.sh
• hack/build-binaries.sh
• hack/build.sh
• hack/linter.sh
• hack/test-all.sh
• hack/test-e2e.sh
• hack/test-external-loop-cert-manager.sh
• hack/test-external.sh
• hack/test.sh
• hack/verify-no-dirty-files.sh

Systems where this is particularly relevant:

• NixOS: Bash is located at /run/current-system/sw/bin/bash
• Some BSD systems: 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 for portable shell scripts and is already widely adopted in the Go ecosystem (many vendor scripts already use this pattern).

Environment:

• kapp version: N/A (affects repository scripts, not runtime)
• OS: Any non-standard Unix-like system (NixOS, BSD variants, etc.)
• Kubernetes version: N/A (repository/build issue)