Fix golang version during build flow

**What**
- Fix copy-pasta error and use Go 1.15 for both the build and publish
  workflows

Signed-off-by: Lucas Roesler <[email protected]>

Author: LucasRoesler

.github/workflows/build.yaml

@@ -12,7 +12,7 @@ jobs:
   build:
     strategy:
       matrix:
-        go-version: [ 1.13.x ]
+        go-version: [ 1.15.x ]
         os: [ ubuntu-latest ]
     runs-on: ${{ matrix.os }}
     steps: