ci: regtests: build own libsecp256k1 instead of using apt

- version in apt is too old (cirrus is using ubuntu 22.04 LTS atm) for schnorr module
- this way we have better control of exact version to use

.cirrus.yml

@@ -99,7 +99,8 @@ task:
     populate_script: mkdir -p /tmp/bitcoind
   install_script:
     - apt-get update
-    - apt-get -y install libsecp256k1-dev curl jq bc
+    - apt-get -y install curl jq bc
+    # install electrum
     - pip3 install .[tests]
     # install e-x some commits after 1.16.0 tag
     - pip3 install git+https://github.com/spesmilo/electrumx.git@4e66804dc0d668cd6bd4602b547e2f5b2e227e97
@@ -109,6 +110,16 @@ task:
     - BITCOIND_URL=https://bitcoincore.org/bin/bitcoin-core-$BITCOIND_VERSION/$BITCOIND_FILENAME
     - tar -xaf $BITCOIND_PATH || (rm -f /tmp/bitcoind/* && curl --output $BITCOIND_PATH $BITCOIND_URL && tar -xaf $BITCOIND_PATH)
     - cp -a bitcoin-$BITCOIND_VERSION/* /usr/
+  libsecp_build_cache:
+    folder: contrib/_saved_secp256k1_build
+    fingerprint_script: sha256sum ./contrib/make_libsecp256k1.sh
+    populate_script:
+      - apt-get -y install automake libtool
+      - ./contrib/make_libsecp256k1.sh
+      - mkdir contrib/_saved_secp256k1_build
+      - cp electrum/libsecp256k1.so.* contrib/_saved_secp256k1_build/
+  libsecp_install_script:
+    - cp contrib/_saved_secp256k1_build/* electrum/
   bitcoind_service_background_script:
     - tests/regtest/run_bitcoind.sh
   electrumx_service_background_script: