CI: Use forward-slashes in paths (cross-platform)

Backslashes "\" in these paths can be interpreted as escape characters
on Unix (Linux, macOS). Replace with forward-slashes, "/",
which are interpreted the same (as subdirectory indicator) everywhere.

Author: DeeDeeG

script/vsts/nightly-release.yml

@@ -4,7 +4,7 @@ jobs:
       # This has to be done separately because VSTS inexplicably
       # exits the script block after `npm install` completes.
       - script: |
-          cd script\vsts
+          cd script/vsts
           npm install
         displayName: npm install
       - script: node script\vsts\get-release-version.js --nightly

script/vsts/pull-requests.yml

@@ -6,7 +6,7 @@ jobs:
       # This has to be done separately because VSTS inexplicably
       # exits the script block after `npm install` completes.
       - script: |
-          cd script\vsts
+          cd script/vsts
           npm install
         displayName: npm install
       - script: node script\vsts\get-release-version.js

script/vsts/release-branch-build.yml

@@ -9,7 +9,7 @@ jobs:
       # This has to be done separately because VSTS inexplicably
       # exits the script block after `npm install` completes.
       - script: |
-          cd script\vsts
+          cd script/vsts
           npm install
         displayName: npm install
       - script: node script\vsts\get-release-version.js