fix(.github/workflow): strict naming convention for changed rules files

[leogr]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind feature

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area rules

/area registry

/area build

/area documentation

Proposed rule maturity level

Uncomment one (or more) /area <> lines (only for PRs that add or modify rules):

/area maturity-stable

/area maturity-incubating

/area maturity-sandbox

/area maturity-deprecated

What this PR does / why we need it:

This PR aims to address the CI problem discovered in this PR.

The "Validate rules file" job wrongly assumed all files under /rules :point_down. However, we have to deal with the possibility that other legitimate files may be stored under that folder.

2024-05-20T14:20:44+0000: Falco version: 0.38.0-275+dfbd181 (x86_64)
2024-05-20T14:20:44+0000: CLI args: /usr/bin/falco -o json_output=true -V rules/OWNERS -o log_level=debug -o log_stderr=true -o log_syslog=false -o stdout_output.enabled=true
2024-05-20T14:20:44+0000: Falco initialized with configuration files:
2024-05-20T14:20:44+0000:    /etc/falco/falco.yaml
2024-05-20T14:20:44+0000: System info: Linux version 6.5.0-1021-azure (buildd@lcy02-amd[64](https://github.com/falcosecurity/rules/actions/runs/9160120617/job/25182147619?pr=247#step:6:65)-097) (x86_64-linux-gnu-gcc-11 (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, GNU ld (GNU Binutils for Ubuntu) 2.38) #22~22.04.1-Ubuntu SMP Tue Apr 30 16:08:18 UTC 2024
2024-05-20T14:20:44+0000: Validating rules file(s):
2024-05-20T14:20:44+0000:    rules/OWNERS
2024-05-20T14:20:44+0000: closing inspectors
Error: rules/OWNERS: Invalid
1 Errors:
In rules content: (rules/OWNERS:0:0)
------
approvers:
^
------
LOAD_ERR_YAML_VALIDATE (Error validating internal structure of YAML file): Rules content is not yaml array of objects

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[darryk10]

LGTM!

Thanks Leo

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: darryk10, leogr, LucaGuerra

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