feat(CI): use new variable syntax.

Author: aensidhe

.azure/tests.yml

@@ -3,59 +3,59 @@ steps:
   displayName: dotnet build
   inputs:
     command: build
-    projects: '{{ sln }}'
+    projects: $(sln)
     arguments: -c Release
     verbosityRestore: minimal
 
 - ${{ if eq(variables['Agent.OS'], 'Windows_NT') }}:
   - template: test.yml
     parameters:
-      path: '{{ tests }}'
+      path: $(tests)
       framework: net46
 
   - template: test.yml
     parameters:
-      path: '{{ tests }}'
+      path: $(tests)
       framework: net461
 
   - template: test.yml
     parameters:
-      path: '{{ tests }}'
+      path: $(tests)
       framework: net462
 
   - template: test.yml
     parameters:
-      path: '{{ tests }}'
+      path: $(tests)
       framework: net47
 
   - template: test.yml
     parameters:
-      path: '{{ tests }}'
+      path: $(tests)
       framework: net471
 
 - template: test.yml
   parameters:
-    path: '{{ tests }}'
+    path: $(tests)
     framework: netcoreapp1.0
     frameworkGlobal: false
 
 - template: test.yml
   parameters:
-    path: '{{ tests }}'
+    path: $(tests)
     framework: netcoreapp1.1
     frameworkGlobal: false
 
 - template: test.yml
   parameters:
-    path: '{{ tests }}'
+    path: $(tests)
     framework: netcoreapp2.0
 
 - template: test.yml
   parameters:
-    path: '{{ tests }}'
+    path: $(tests)
     framework: netcoreapp2.1
 
 - template: test.yml
   parameters:
-    path: '{{ tests }}'
+    path: $(tests)
     framework: netcoreapp2.2