configure default chain via env var

[InoMurko]

What it solves

Resolves #
Configurable default chain via env var.

How this PR fixes it

Reads the env var!

How to test it

Screenshots

[github-actions]

CLA Assistant Lite:
Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger the CLA Action by commenting recheckcla in this Pull Request}

[coveralls]

Pull Request Test Coverage Report for Build 1729066759

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 65 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.006%) to 31.995%

---

Files with Coverage Reduction	New Missed Lines	%
src/routes/safe/components/Apps/utils.ts	6	19.1%
src/routes/safe/components/Apps/hooks/useIframeMessageHandler.ts	26	0%
src/routes/safe/components/Apps/components/AppFrame.tsx	33	0%

Totals
Change from base Build 1728559105:	-0.006%
Covered Lines:	3115
Relevant Lines:	8667

---

💛 - Coveralls

[InoMurko]

I have read the CLA Document and I hereby sign the CLA

[katspaugh]

Thanks! Looks good 👍

Could you also add your GitHub handle to this file? https://github.com/gnosis/safe-react/blob/dev/.github/workflows/cla.yml#L24

[katspaugh]

Thank you @InoMurko!