fix: update template variable for extra index URLs in init file

[noamgot]

fixes #4000

FYI @nichmor

I didn't add tests - not sure where/how to add them. Feel free to add them...

--
Here is what I did locally to test this (pixid is the compiled version of my local repo):

❯ cat $PIXI_HOME/config.toml
[pypi-config]
extra-index-urls = ["http://localhost:9999/simple/"]

 # create projects:
❯ pixi init test-before
✔ Created $HOME/test-before/pixi.toml
❯ pixid init test-after
✔ Created $HOME/test-after/pixi.toml

# examine pixi.toml files:
# before:
❯ cat test-before/pixi.toml

[workspace]
channels = ["conda-forge"]
name = "test-before"
platforms = ["linux-64"]
version = "0.1.0"

[tasks]

[dependencies]

# after:
❯ cat test-after/pixi.toml

[workspace]
channels = ["conda-forge"]
name = "test-after"
platforms = ["linux-64"]
version = "0.1.0"

[pypi-options]

extra-index-urls = ["http://localhost:9999/simple/"]

[tasks]

[dependencies]

[ruben-arts]

Testing this is a bit of a weird one as we don't want to set global config in test tests. But it so simple that I'm just going to merge this!