gpg-agent: fix compatibility with sh when enableSshSupport

[DDoSolitary]

Description

When services.gpg-agent.enableSshSupport is set, the module adds script to home.sessionVariablesExtra with non-portable double bracket [[ syntax.

The script is written to hm-session-vars.sh, which will be referenced by ~/.profile (if managed by home-manager) and sourced by sh which doesn't support this syntax. (e.g. when logging in graphical session with uwsm, it uses sh to source ~/.profile to get environment variables)

The portable single bracket [ syntax also supports the -z operator, so we can just use it instead.

Checklist

• Change is backwards compatible.

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Maintainer CC

@rycee