Fix codecov by calling the new binary paths from rust 1.45

Rustc 1.45 moved the paths to test binaries, so we need to update
our CI scripts to run the correct ones under kcov.

The solution to this was pointed out by Val at
https://github.com/rust-bitcoin/rust-lightning/pull/774#issuecomment-763250623

.github/workflows/build.yml

@@ -75,7 +75,7 @@ jobs:
       - name: Generate coverage report
         if: matrix.coverage
         run: |
-          for file in target/debug/lightning-*; do
+          for file in target/debug/deps/lightning*; do
             [ -x "${file}" ] || continue;
             mkdir -p "target/cov/$(basename $file)";
             ./kcov-build/usr/local/bin/kcov --exclude-pattern=/.cargo,/usr/lib --verify "target/cov/$(basename $file)" "$file";