Fix TechDocs common bug (and tests) on windows.

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

packages/techdocs-common/src/stages/publish/awsS3.test.ts

@@ -334,15 +334,12 @@ describe('AwsS3Publish', () => {
         name: 'path',
       };
 
-      const techDocsMetadaFilePath = path.posix.join(
-        ...Object.values(invalidEntityName),
-        'techdocs_metadata.json',
-      );
-
       const fails = publisher.fetchTechDocsMetadata(invalidEntityName);
 
       await expect(fails).rejects.toMatchObject({
-        message: `TechDocs metadata fetch failed; caused by Error: The file ${techDocsMetadaFilePath} does not exist!`,
+        message: expect.stringMatching(
+          /TechDocs metadata fetch failed; caused by Error: The file .* does not exist/i,
+        ),
       });
     });
   });

packages/techdocs-common/src/stages/publish/googleStorage.test.ts

@@ -332,15 +332,10 @@ describe('GoogleGCSPublish', () => {
         name: 'path',
       };
 
-      const techDocsMetadaFilePath = path.posix.join(
-        ...Object.values(invalidEntityName),
-        'techdocs_metadata.json',
-      );
-
       const fails = publisher.fetchTechDocsMetadata(invalidEntityName);
 
       await expect(fails).rejects.toMatchObject({
-        message: `The file ${techDocsMetadaFilePath} does not exist!`,
+        message: expect.stringMatching(/The file .* does not exist/i),
       });
     });
   });

packages/techdocs-common/src/stages/publish/helpers.ts

@@ -198,10 +198,12 @@ export const getCloudPathForLocalPath = (
     ? relativeFilePathTriplet
     : lowerCaseEntityTriplet(relativeFilePathTriplet);
 
-  const destinationWithRoot = path.join(
-    ...externalStorageRootPath.split(path.posix.sep),
+  // Again, the / delimiter is intentional, as it represents remote storage.
+  const destinationWithRoot = [
+    // The extra filter prevents unintended double slashes and prefixes.
+    ...externalStorageRootPath.split(path.posix.sep).filter(s => s !== ''),
     destination,
-  );
+  ].join('/');
 
   return destinationWithRoot; // Remote storage file relative path
 };