GitHub: use bitcoind v26.0 for CI, remove default value

To make it less confusing what version of bitcoind is actually
installed, we now require the version to be specified as a command line
argument.

Because we tag the version in the docker image tag as bitcoin-core:XX
but the binary internally is located under /opt/bitcoin-XX.Y/, we
manually set Y to 0.

.github/workflows/main.yml

@@ -21,7 +21,7 @@ defaults:
     shell: bash
 
 env:
-  BITCOIN_VERSION: "23.0"
+  BITCOIN_VERSION: "26"
 
   # If you change this value, please change it in the following files as well:
   # /.travis.yml
@@ -215,7 +215,7 @@ jobs:
           key-prefix: unit-test
 
       - name: install bitcoind
-        run: ./scripts/install_bitcoind.sh
+        run: ./scripts/install_bitcoind.sh $BITCOIN_VERSION
 
       - name: run ${{ matrix.unit_type }}
         run: make log="stdlog debug" ${{ matrix.unit_type }}
@@ -271,7 +271,7 @@ jobs:
           key-prefix: integration-test
 
       - name: install bitcoind
-        run: ./scripts/install_bitcoind.sh
+        run: ./scripts/install_bitcoind.sh $BITCOIN_VERSION
 
       - name: run ${{ matrix.name }}
         run: make itest-parallel ${{ matrix.args }}