Remove useless empty shell variable checks

[nikobockerman]

These were initially added when shellcheck rules still required validating all environment variables. Since then capitalized environment variables are allowed with shellcheck to avoid bloated checks.

[Copilot]

Pull request overview

This pull request removes redundant environment variable validation checks from GitHub Actions shell scripts. These explicit checks are no longer needed because shellcheck rules have been updated to allow capitalized environment variables without requiring validation, and the scripts use set -u which causes immediate exit on undefined variable references.

Changes:

• Removed validation checks for GitHub Actions environment variables (GITHUB_ENV, GITHUB_OUTPUT, RUNNER_OS, RUNNER_TEMP, GITHUB_WORKSPACE)
• Removed validation checks for input variables (CACHE_MODE, MISE_VERSION)
• Simplified the "Validate inputs" section to rely on bash's built-in undefined variable detection

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/actions/uv-project-setup/init.sh	Removed redundant validation checks for GITHUB_ENV, GITHUB_OUTPUT, RUNNER_OS, RUNNER_TEMP, and CACHE_MODE
.github/actions/mise-project-setup/init.sh	Removed redundant validation checks for GITHUB_ENV, GITHUB_OUTPUT, RUNNER_OS, RUNNER_TEMP, MISE_VERSION, and CACHE_MODE

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