fix(packaging): use sysconfig environment file in RPMs (#26076)

[inflatador]

In RHEL-based Linux distros, service specific environment variable files typically live in /etc/sysconfig (as opposed to /etc/default in Debian).

Vector's RPM currently uses the Debian path in its systemd unit file, which can lead to config not being loaded properly. Avoid this by switching to the RHEL convention.

References

Closes: #26076

Vector configuration

N/A

How did you test this PR?

1. Install the RPM on a RHEL-based OS (I used Fedora 44)
2. Copy /etc/default/vector to /etc/sysconfig/vector
3. Create a systemd override at /etc/systemd/system/vector.service.d/vector-envvars.conf with the following content:

[Service]
EnvironmentFile=-
EnvironmentFile=-/etc/sysconfig/vector

4)Reload systemd systemctl daemon-reload
5) Restart the vector server systemctl restart vector.service
6) Verify the service starts cleanly systemctl status vector.service
7)Verify the service is using your override systemctl cat vector.service

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

Y

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

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[chatgpt-codex-connector]

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[inflatador]

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