.htaccess redirect loop when trying to add forced HTTPS rule

.htaccess redirect loop when trying to add forced HTTPS rule

I started receiving this error after trying to incorporate a rule to force
HTTPS in the production environment. The BWC_ENV environment variable can
have a handful of different values: "prod", "stage", "ben_local",
"nam_local", etc.
Here's my .htaccess:
RewriteEngine On
# Parse the subdomain as a variable we can access in our scripts
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !robots.txt
RewriteCond %{HTTP_HOST} !^www
RewriteCond %{HTTP_HOST} ^([^\.]+)\.([^\.]+)\.([^\.]+)$
RewriteRule ^(.*)$ /$1?subdomain=%1
# Ditto for the path; map all requests to /index.php
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !robots.txt
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /index.php?path=$1 [L,QSA]
# robots.txt - supply the correct one for each environment
RewriteRule ^robots.txt$ /robots.prod.txt [NC]
RewriteCond %{ENV:BWC_ENV} !prod
RewriteRule ^robots.prod.txt$ /robots.stage.txt [NC]
# Force HTTPS
RewriteCond %{HTTPS} !=on
RewriteCond %{ENV:BWC_ENV} ^prod$
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]