GODRIVER-1911 Fix Windows/macos test failures for CSFLE (#603)

This commit addresses two failures that started showing up after GODRIVER-1855:

- Set the PYTHON environment variable to point to the local python3 installation before calling set-temp-creds.sh
- For Windows tasks, update the libmongocrypt URL to install the latest stable version (v1.2.0)

.evergreen/config.yml

@@ -75,8 +75,7 @@ functions:
           if [ "Windows_NT" = "$OS" ]; then
              mkdir -p c:/libmongocrypt/include
              mkdir -p c:/libmongocrypt/bin
-             # TODO: After a stable libmongocrypt 1.1.0 is released in MONGOCRYPT-293, update this URL to: https://s3.amazonaws.com/mciuploads/libmongocrypt/windows/latest_release/libmongocrypt.tar.gz
-             curl https://s3.amazonaws.com/mciuploads/libmongocrypt/windows/latest_release/libmongocrypt_unstable.tar.gz --output libmongocrypt.tar.gz
+             curl https://s3.amazonaws.com/mciuploads/libmongocrypt/windows/latest_release/libmongocrypt.tar.gz --output libmongocrypt.tar.gz
              tar -xvzf libmongocrypt.tar.gz
              cp ./bin/mongocrypt.dll c:/libmongocrypt/bin
              cp ./include/mongocrypt/*.h c:/libmongocrypt/include
@@ -303,6 +302,15 @@ functions:
             export AWS_SECRET_ACCESS_KEY="${cse_aws_secret_access_key}"
             export AWS_DEFAULT_REGION="us-east-1"
             ${PYTHON3_BINARY} -m venv ./venv
+
+            # Set the PYTHON environment variable to point to the active python3 binary. This is used by the
+            # set-temp-creds.sh script.
+            if [ "Windows_NT" = "$OS" ]; then
+              export PYTHON="$(pwd)/venv/Scripts/python"
+            else
+              export PYTHON="$(pwd)/venv/bin/python"
+            fi
+
             ./venv/${VENV_BIN_DIR|bin}/pip3 install boto3
             . ${DRIVERS_TOOLS}/.evergreen/csfle/set-temp-creds.sh
           fi