Bug 1594520 - Use full path to mach in .vscode/tasks.json on Windows r=jya

Current versions of mozilla-build ignore the current working directory
so use full path to mach. Due to path mangling issues copying from
vscode -> powershell -> msys-bash, I used a PowerShell fragment to
translate the path.

Differential Revision: https://phabricator.services.mozilla.com/D52084

--HG--
extra : moz-landing-system : lando

.vscode/tasks.json

@@ -4,10 +4,15 @@
     "version": "2.0.0",
     "type": "shell",
     "command": "${workspaceFolder}/mach",
-    "windows": {
-      "command": "\"\\mozilla-build\\start-shell.bat mach\""
-    },
     "args": ["--log-no-times"],
+    "windows": {
+      "command": "/mozilla-build/start-shell.bat",
+      "args": [
+        // Use PowerShell to mangle path for mozilla-build environment
+        {"value": "$('${workspaceFolder}\\mach' -replace '\\\\','/')", "quoting": "weak"},
+        "--log-no-times"
+      ]
+    },
     "tasks": [
       {
         "label": "clobber",