Travis+GitHub: update to bitcoind 0.20.1, remove cache

With this commit we use the same version of bitcoind in all tests and
also remove the caches for the binary as docker has its own cache.

.github/workflows/main.yml

@@ -17,7 +17,7 @@ env:
   GOCACHE: /home/runner/work/go/pkg/build
   GOPATH: /home/runner/work/go
   DOWNLOAD_CACHE: /home/runner/work/download_cache
-  BITCOIN_VERSION: 0.19.1
+  BITCOIN_VERSION: 0.20.1
   GO_VERSION: 1.15.2
 
 jobs:
@@ -236,14 +236,6 @@ jobs:
         with:
           go-version: '~${{ env.GO_VERSION }}'
 
-      - name: bitcoin cache
-        uses: actions/cache@v1
-        with:
-          path: /home/runner/bitcoin/bitcoin-${{ env.BITCOIN_VERSION }}/bin
-          key: lnd-${{ runner.os }}-bitcoin-${{ env.BITCOIN_VERSION }}
-          restore-keys: |
-            lnd-${{ runner.os }}-bitcoin-${{ env.BITCOIN_VERSION }}
-
       - name: install bitcoind
         run: ./scripts/install_bitcoind.sh