Set Node.js version to 18.15.0 (#3633)

We want to prevent the problem with generation of artifacts during
execution of the `hardhat compile` command (part of `yarn deploy`). We
saw in other project that sometimes command was not not generating
expected artifacts. The problem is caused by the process silently
quitting, which is related to the used version of Node (as described in
NomicFoundation/hardhat#3877). We've confirmed
the problem is reproducible on `v18.16.0`, now we're downgrading to
`v18.15.0` to fix the issue.

Refs:
#3595
threshold-network/solidity-contracts#145

.github/workflows/npm-ecdsa.yml

@@ -25,7 +25,10 @@ jobs:
 
       - uses: actions/setup-node@v3
         with:
-          node-version: "18.x"
+          # Using fixed version, because 18.16 may cause issues with the
+          # artifacts generation during `hardhat compile` - see
+          # https://github.com/NomicFoundation/hardhat/issues/3877.
+          node-version: "18.15.0"
           registry-url: "https://registry.npmjs.org"
           cache: "yarn"
           cache-dependency-path: solidity/ecdsa/yarn.lock

.github/workflows/npm-random-beacon.yml

@@ -25,7 +25,10 @@ jobs:
 
       - uses: actions/setup-node@v3
         with:
-          node-version: "18.x"
+          # Using fixed version, because 18.16 may cause issues with the
+          # artifacts generation during `hardhat compile` - see
+          # https://github.com/NomicFoundation/hardhat/issues/3877.
+          node-version: "18.15.0"
           registry-url: "https://registry.npmjs.org"
           cache: "yarn"
           cache-dependency-path: solidity/random-beacon/yarn.lock