Bug 1594520 - Change directory before running mach in .vscode/tasks.j…

…son on Windows. r=jya

The build is quite fragile if the path to mach is not precisely
formatted. Work around this by cd-ing to the appropriate directory and
then calling 'mach' with a relative path. This mimics a normal user
workflow in mozilla-build.

Differential Revision: https://phabricator.services.mozilla.com/D52968

.vscode/tasks.json

@@ -8,8 +8,12 @@
     "windows": {
       "command": "/mozilla-build/start-shell.bat",
       "args": [
+        "cd",
         // Use PowerShell to mangle path for mozilla-build environment
-        {"value": "$('${workspaceFolder}\\mach' -replace '\\\\','/')", "quoting": "weak"},
+        {"value": "$('${workspaceFolder}' -replace '\\\\','/')", "quoting": "weak"},
+        "';'",
+
+        "mach",
         "--log-no-times"
       ]
     },