docs: add BREV_SKILL_BRANCH env var to INSTALLATION.md

Document the env var option for installing from a specific branch
when using the `brev claude-skill` CLI command.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: theFong

.claude/skills/brev-cli/INSTALLATION.md

@@ -26,10 +26,16 @@ curl -fsSL https://raw.githubusercontent.com/brevdev/brev-cli/main/scripts/insta
 
 ### Install from a specific branch
 
+**Using the standalone script:**
 ```bash
 curl -fsSL https://raw.githubusercontent.com/brevdev/brev-cli/main/scripts/install-claude-skill.sh | bash -s -- --branch my-branch
 ```
 
+**Using the CLI command:**
+```bash
+BREV_SKILL_BRANCH=my-branch brev claude-skill
+```
+
 ### Uninstall
 
 ```bash