Add an environment variable for the support key

As a followup from change I75762ad620132037523fa82167a3ff17075c7027, it
would be nice to be able to set the support key via environment
variables. This commit adds support for a new 'support_key' environment
variable

Signed-off-by: Skyler Grey <skyler.grey@collabora.com>
Change-Id: Ibfe2bf055b34e4ae01a42b5616f6ad7977f15157

wsd/COOLWSD.cpp

@@ -3037,6 +3037,7 @@ void COOLWSD::initializeEnvOptions()
     if ((optionValue = std::getenv("password")) != nullptr) _overrideSettings["admin_console.password"] = optionValue;
     if ((optionValue = std::getenv("server_name")) != nullptr) _overrideSettings["server_name"] = optionValue;
     if ((optionValue = std::getenv("dictionaries")) != nullptr) _overrideSettings["allowed_languages"] = optionValue;
+    if ((optionValue = std::getenv("support_key")) != nullptr) _overrideSettings["support_key"] = optionValue;
 }
 
 #if !MOBILEAPP