bug#36074: Incorrect quoting of GUIX_LOCPATH environment variable in guix-daemon.service

[Jack Hill]

Hi Guix,

Based on my experience setting up Guix on a CentOS 7 foreign distribution, 
if quoted, the variable name in a systemd unit Environment definition 
should be inside the quotes. I didn't not find definitive documentation 
stating this, but all the examples show it this way. If the value is left 
like

```
Environment=GUIX_LOCPATH='/path/to/profile/lib/locale'
```

the guix-daemon process is not able to find the right locale even when it 
is installed.

Best,
Jack