Merge bitcoin#18958: guix: Make V=1 more powerful for debugging

f852761 guix: Add clarifying documentation for V env var (Carl Dong)
85f4a4b guix: Make V=1 more powerful for debugging (Carl Dong)

Pull request description:

  ```
  - Print commands in both unexpanded and expanded forms
  - Set VERBOSE=1 for CMake
  ```

  Ping MarcoFalke hopefully you use `V=1` already for the Guix builds on DrahtBot?

ACKs for top commit:
  fanquake:
    ACK f852761. Ran a Windows Guix build and compared the output from master and this PR when using `V=1`. i.e `HOSTS=x86_64-w64-mingw32 PATH="/root/.config/guix/current/bin${PATH:+:}$PATH" V=1 ./contrib/guix/guix-build.sh`.

Tree-SHA512: 8bc466fa7b869618bbd5a0a91c6b23d4785009289f8dfb93b0349317463a9ab9ece128c72436e02a0819722a63e703100aed15807867a716fda891292fcb9d9d

contrib/guix/README.md

@@ -142,6 +142,11 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
   If non-empty, will pass `V=1` to all `make` invocations, making `make` output
   verbose.
 
+  Note that any given value is ignored. The variable is only checked for
+  emptiness. More concretely, this means that `V=` (setting `V` to the empty
+  string) is interpreted the same way as not setting `V` at all, and that `V=0`
+  has the same effect as `V=1`.
+
 * _**ADDITIONAL_GUIX_ENVIRONMENT_FLAGS**_
 
   Additional flags to be passed to `guix environment`. For a fully-bootstrapped

contrib/guix/libexec/build.sh

@@ -3,6 +3,14 @@ export LC_ALL=C
 set -e -o pipefail
 export TZ=UTC
 
+if [ -n "$V" ]; then
+    # Print both unexpanded (-v) and expanded (-x) forms of commands as they are
+    # read from this file.
+    set -vx
+    # Set VERBOSE for CMake-based builds
+    export VERBOSE="$V"
+fi
+
 # Check that environment variables assumed to be set by the environment are set
 echo "Building for platform triple ${HOST:?not set} with reference timestamp ${SOURCE_DATE_EPOCH:?not set}..."
 echo "At most ${MAX_JOBS:?not set} jobs will run at once..."