Fix make image targets

[dhurley]

Proposed changes

• Update make image & make oss-image to take a NGINX_AGENT_VERSION argument
• Update make oss-image to take a IMAGE_BUILD_TARGET argument
• Fix postinstall script error when running on SUSE OS (awk wasn't installed so I used cut instead)

Can now call make targets like this now:

IMAGE_BUILD_TARGET=install-agent-repo NGINX_AGENT_VERSION=2.37.0 OS_RELEASE=alpine OS_VERSION=3.20 make oss-image

NGINX_AGENT_VERSION=2.37.0 OS_RELEASE=alpine OS_VERSION=3.20 make image

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13