nixos/grafana-agent: fix shellcheck

[sedlund]

Nixpkgs version

• Unstable (25.05)

Describe the bug

with systemd.enableStrictShellChecks = true; I get

unit-script-grafana-agent-start> export HOSTNAME=$(< /proc/sys/kernel/hostname)
unit-script-grafana-agent-start>        ^------^ SC2155 (warning): Declare and assign separately to avoid masking return values.

easy enough fix, but the comment above I'm not sure is true (any longer?):

        # We can't use Environment=HOSTNAME=%H, as it doesn't include the domain part.

systemd.unit(5) states that %H is the host name and %l is the short name (truncated to first dot to remove any domain component`)

So I'm opening the issue to solicit feedback on which way this should go and any glean any further context available.

Steps to reproduce

enable systemd.enableStrictShellChecks

Expected behaviour

enableStrictShellChecks should work

Screenshots

No response

Relevant log output

Additional context

No response

System metadata

this path will be fetched (0.02 MiB download, 0.06 MiB unpacked):
/nix/store/md93ycspqdq3arrv9ms0m4c33jssizgg-stdenv-linux
copying path '/nix/store/md93ycspqdq3arrv9ms0m4c33jssizgg-stdenv-linux' from 'https://cache.nixos.org'...

• system: "x86_64-linux"
• host os: Linux 5.15.167.4-microsoft-standard-WSL2, NixOS, 25.05 (Warbler), 25.05.20250319.a84ebe2
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.24.12
• nixpkgs: /nix/store/1728d3jg85mkz2w2cvk6vi74i30fn6x7-source

Notify maintainers

@flokli
@zimbatm

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[sedlund]

closing this, as I see grafana-agent is going to be end-of-life 2025-11-01 for grafana-alloy