Ability to disable HTTPS_PROXY environment variable always being passed through.

[wkpatrick]

Clear and concise description of the problem

One of the libraries in my project attempts to use a proxy if HTTPS_PROXY is set as a variable, even if it is just blank.

Suggested solution

Possible solutions discussed in the matrix room by 6543:

"Well we just either need to add a option to
https://github.com/woodpecker-ci/woodpecker/blob/main/pipeline/frontend/yaml/compiler/option.go#L219
and expose it as flag or add a check to
https://github.com/woodpecker-ci/woodpecker/blob/main/pipeline/frontend/yaml/compiler/option.go#L228
If all 3 are "" and if so ignore it
I'm for second one ..."

Alternative

No response

Additional context

No response

Validations

• Checked that the feature isn't part of the next version already [https://woodpecker-ci.org/faq#which-version-of-woodpecker-should-i-use]
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.