ci: another take at fixing toolstate

Seems like the variable showed by $(ciCheckoutPath) on Azure Pipelines
was wrong, making the toolstate script fail. This commit changes that
function to return the variable previously used by the toolstate script.
Other uses of the function were audited, and there should be no
conflict.

Author: pietroalbini

src/ci/shared.sh

@@ -80,7 +80,7 @@ function ciCommit {
 
 function ciCheckoutPath {
     if isAzurePipelines; then
-        echo "${SYSTEM_WORKFOLDER}"
+        echo "${BUILD_SOURCESDIRECTORY}"
     elif isGitHubActions; then
         echo "${GITHUB_WORKSPACE}"
     else