bump pxt-core to hopefully fix karma tests (#211)

* bump pxt-core to hopefully fix karma tests

* bump again

* update node version

.github/workflows/pxt-buildmain.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [8.x]
+        node-version: [18.x]
 
     steps:
       - uses: actions/checkout@v1

.github/workflows/pxt-buildpr.yml

@@ -9,7 +9,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [8.x]
+        node-version: [18.x]
 
     steps:
       - uses: actions/checkout@v1

.github/workflows/pxt-buildpush.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [8.x]
+        node-version: [18.x]
 
     steps:
       - uses: actions/checkout@v1