Set the `TERM` env variable (#3420)

Resolves #3419

I have tested this change with all the pagers shown in [the
docs](https://github.com/jesseduffield/lazygit/blob/master/docs/Custom_Pagers.md).
Are there others that people frequently use and I should test?

A nice side effect of setting `TERM=dumb` is that `less` now correctly
complains (e.g. when forgetting to set `--paging=never` for delta:


![image](https://github.com/jesseduffield/lazygit/assets/4602612/33e9c048-6ab0-4196-95f6-86ee8effc873)


## Pagers Tested

### none

![image](https://github.com/jesseduffield/lazygit/assets/4602612/4e408fe6-5f19-4142-9183-de56fd738962)

### Delta

![image](https://github.com/jesseduffield/lazygit/assets/4602612/90dd12fa-ed58-4d49-9f71-da5e57f63a74)

```yaml
git:
  paging:
    colorArg: always
    pager: delta --paging=never
```

#### diff-so-fancy

![image](https://github.com/jesseduffield/lazygit/assets/4602612/930e6b93-904e-49a2-bfc2-9b5d9639f514)

```yaml
git:
  paging:
    colorArg: always
    pager: diff-so-fancy
```

#### ydiff

![image](https://github.com/jesseduffield/lazygit/assets/4602612/c2464f0c-f34f-4ebc-9624-8bc0350d92a7)

```yaml
git:
  paging:
    colorArg: always
    pager: ydiff -p cat
```

#### difft

![image](https://github.com/jesseduffield/lazygit/assets/4602612/599dee7a-6568-40e4-9213-76ba643d651f)

```yaml
git:
  paging:
    externalDiffCommand: difft --color=always
```

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