INT-1280 fix Windows signing

Make the signtool.exe sitting in our current working directory executable.
Remove the copy to electron-winstaller's vendor directory.

In Squirrel.Windows/src/Update/Program.cs, function signPEFile:
search order for "signtool.exe" is:

- first: ".\signtool.exe"
- second: GetExecutingAssembly().Location
- "Run down PATH and hope for the best"

source: https://github.com/Squirrel/Squirrel.Windows/blob/b814ada81a0060be22ec2f24efb6be0ff8c16d95/src/Update/Program.cs#L577-L588

Author: kangas

.evergreen.yml

@@ -89,8 +89,7 @@ functions:
           -o "signtool.exe" \
           --url "https://s3.amazonaws.com/boxes.10gen.com/build/signtool.exe"
         rm -f node_modules/electron-winstaller/vendor/signtool.exe
-        cp signtool.exe node_modules/electron-winstaller/vendor/signtool.exe
-        chmod +x node_modules/electron-winstaller/vendor/signtool.exe
+        chmod +x signtool.exe
 
   # Environment variables set for use by notary-service signtool.exe on Windows:
   # NOTARY_URL, NOTARY_AUTH_TOKEN, NOTARY_SIGNING_KEY, NOTARY_SIGNING_COMMENT