fix: fix logic for parsing absolute path to nginx config file

[sylwang]

Proposed changes

Issue

• From the NGINX UI, was unable to publish config changes for an instance when a .conf file was created under /etc/nginx/conf.d. The absolute path used to update the config changes contain variables with $ (e.g., /etc/nginx/$variable_name) and a No such file or directory error occurred.
• Agent log showed errors related to parsing the nginx config files using an absolute path that contains variables.

time="2023-08-16T17:26:14Z" level=info msg="Upload sending done e0991342-0956-430a-a4f5-ece28395c421 (chunks=2)"
time="2023-08-16T17:38:42Z" level=warning msg="Error unable to add file watcher for /etc/nginx/conf.d/.config1.conf.swx : stat /etc/nginx/conf.d/.config1.conf.swx: no such file or directory"
time="2023-08-16T17:38:42Z" level=error msg="Error occurred adding watcher for /etc/nginx/conf.d/.config1.conf.swx: stat /etc/nginx/conf.d/.config1.conf.swx: no such file or directory"
time="2023-08-16T17:38:44Z" level=info msg="Upload: Sending data chunk data 0 (messageId=22aca969-1940-4e38-9a0d-a9112917673a)"
time="2023-08-16T17:38:44Z" level=info msg="Upload: Sending data chunk data 1 (messageId=22aca969-1940-4e38-9a0d-a9112917673a)"
time="2023-08-16T17:38:44Z" level=info msg="Upload sending done 22aca969-1940-4e38-9a0d-a9112917673a (chunks=2)"
time="2023-08-16T17:40:10Z" level=warning msg="Error unable to add file watcher for /etc/nginx/conf.d/.config1.conf.swx : stat /etc/nginx/conf.d/.config1.conf.swx: no such file or directory"
time="2023-08-16T17:40:10Z" level=error msg="Error occurred adding watcher for /etc/nginx/conf.d/.config1.conf.swx: stat /etc/nginx/conf.d/.config1.conf.swx: no such file or directory"
time="2023-08-16T17:40:12Z" level=error msg="Unable to read nginx config /etc/nginx/nginx.conf: error assemble payload from /etc/nginx/nginx.conf, error: configs: failed to update nginx config: stat /etc/nginx/$repo_crt: no such file or directory"
time="2023-08-16T17:40:12Z" level=warning msg="Unable to upload config for nginx instance b636d4376dea15405589692d3c5d3869ff3a9b26b0e7bb4bb1aa7e658ace1437, error assemble payload from /etc/nginx/nginx.conf, error: configs: failed to update nginx config: stat /etc/nginx/$repo_crt: no such file or directory"

Fix

• Modify updateNginxConfigWithCert so that it correctly checks for the case when an absolute path to a file contains $

Testing

• Recompiled the changes for the Agent and restarted the Agent on the instance, and config changes were published successfully.
• Agent log looked healthy

ubuntu@ip-172-16-0-236:~$ sudo tail /var/log/nginx-agent/agent.log 
time="2023-08-16T22:12:28Z" level=info msg="Reloading NGINX: /usr/sbin/nginx PID: 4290"
time="2023-08-16T22:12:28Z" level=info msg="NGINX with process Id: 4290 reloaded"
time="2023-08-16T22:12:29Z" level=info msg="Finished monitoring post reload"
time="2023-08-16T22:12:29Z" level=info msg="No errors found in NGINX errors logs after NGINX reload"
time="2023-08-16T22:12:29Z" level=info msg="Upload: Sending data chunk data 0 (messageId=8f442622-77d9-4507-9027-98a094451e36)"
time="2023-08-16T22:12:29Z" level=info msg="Upload: Sending data chunk data 1 (messageId=8f442622-77d9-4507-9027-98a094451e36)"
time="2023-08-16T22:12:29Z" level=info msg="Upload sending done 8f442622-77d9-4507-9027-98a094451e36 (chunks=2)"
time="2023-08-16T22:12:32Z" level=info msg="Upload: Sending data chunk data 0 (messageId=9377b069-36fb-4901-aa6e-ad49465870d4)"
time="2023-08-16T22:12:32Z" level=info msg="Upload: Sending data chunk data 1 (messageId=9377b069-36fb-4901-aa6e-ad49465870d4)"
time="2023-08-16T22:12:32Z" level=info msg="Upload sending done 9377b069-36fb-4901-aa6e-ad49465870d4 (chunks=2)"

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

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