Evaluate HOST, PORT, and PROXY env vars when Dash.run() is invoked

[martian0x80]

Fixes #2902

Made the changes as proposed in #2902. Nothing major, just re-evaluates environment variables when Dash.run() is invoked.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Evaluate the environment variable after Dash.run() is called
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[T4rk1n]

Looks good but also need to set the parameters to None on line 1984-1986

[T4rk1n]

Sorry we lost the default arguments by setting the parameters to None, they need the previous default instead.

Also need to add a changelog entry then it's good to merge.

[T4rk1n]

Suggested change

	host=None,
	port=None,
	host="127.0.0.1",
	port="8050",

[martian0x80]

I was planning to ask the same, because they were going to be overwritten anyway, and defaults were already set. Will make the changes.

[T4rk1n]

Thank you 💃