CSHARP-5001: Pass OS variable to run-plain-auth-tests.sh (#1287)

Author: BorisDog

evergreen/evergreen.yml

@@ -101,7 +101,7 @@ functions:
       params:
         script: |
           ${PREPARE_SHELL}
-          OS=${OS} sh ${PROJECT_DIRECTORY}/evergreen/install-dotnet.sh
+          sh ${PROJECT_DIRECTORY}/evergreen/install-dotnet.sh
 
   prepare-resources:
     - command: shell.exec
@@ -437,7 +437,8 @@ functions:
           MONGODB_URI: ${plain_auth_mongodb_uri}
         script: |
           ${PREPARE_SHELL}
-          . evergreen/run-plain-auth-tests.sh
+          OS=${OS} \
+            evergreen/run-plain-auth-tests.sh
 
   run-performance-tests:
     - command: shell.exec
@@ -738,7 +739,6 @@ functions:
         script: |
           set +x
           ${PREPARE_SHELL}
-          OS=${OS} \
           AUTH=${AUTH} \
           SSL=${SSL} \
           MONGODB_URI="${MONGODB_URI}" \