coin: Introduce a COIN_DOC_CMAKE_ARGS env variable

This can be used to configure extra cmake args for the documentation
build in coin.

Pick-to: 6.8
Change-Id: I11417bee9630aebe0bbc118cd843b1536101da71
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

Author: Gagi2k

coin/instructions/coin_module_test_docs.yaml

@@ -17,7 +17,7 @@ instructions:
 
   - type: EnvironmentVariable
     variableName: COIN_CMAKE_ARGS
-    variableValue: "-DQT_BUILD_TESTS=OFF"
+    variableValue: "-DQT_BUILD_TESTS=OFF {{.Env.COIN_DOC_CMAKE_ARGS}}"
   - type: EnvironmentVariable
     variableName: CONFIGURE_ENV_PREFIX
     variableValue: "{{.Env.ENV_PREFIX}}"