Avoid adding `SENTRY_DOTENV_PATH` if `env.sentry-build-plugin` doens't exist

[lucas-zimerman]

Problem Statement

By default we always set this environment variable, even if the file doesn't exist, leading to the following warning message on user projects
https://github.com/getsentry/sentry-cli/blob/701f4c8af8a39aac323773bdcdfbe0eb8fdc9a37/src/utils/system.rs#L128-L132

This message misleads people to think something is wrong with the SDK. We should only set SENTRY_DOTENV_PATHS when env.sentry-build-plugin exists so users no longer have a warning message on their console log.

Solution Brainstorm

No response

Are you willing to submit a PR?

None

[linear]

RN-277 Avoid adding `SENTRY_DOTENV_PATH` if `env.sentry-build-plugin` doens't exist

[lucas-zimerman]

implemented on #5029