ci: Add and document BASE_BUILD_DIR

Co-Authored-By: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

ci/test/00_setup_env.sh

@@ -8,7 +8,7 @@ export LC_ALL=C.UTF-8
 
 # The root dir.
 # The ci system copies this folder.
-# This is where the build is done (depends and dist).
+# This is where the depends build is done.
 BASE_ROOT_DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )"/../../ >/dev/null 2>&1 && pwd )
 export BASE_ROOT_DIR
 
@@ -49,8 +49,10 @@ export CCACHE_DIR=${CCACHE_DIR:-$BASE_SCRATCH_DIR/.ccache}
 # The depends dir.
 # This folder exists on the ci host and ci guest. Changes are propagated back and forth.
 export DEPENDS_DIR=${DEPENDS_DIR:-$BASE_ROOT_DIR/depends}
-# Folder where the build is done (bin and lib).
+# Folder where the build result is put (bin and lib).
 export BASE_OUTDIR=${BASE_OUTDIR:-$BASE_SCRATCH_DIR/out/$HOST}
+# Folder where the build is done (dist and out-of-tree build).
+export BASE_BUILD_DIR=${BASE_BUILD_DIR:-$BASE_SCRATCH_DIR/build}
 export PREVIOUS_RELEASES_DIR=${PREVIOUS_RELEASES_DIR:-$BASE_ROOT_DIR/releases/$HOST}
 export SDK_URL=${SDK_URL:-https://bitcoincore.org/depends-sources/sdks}
 export DOCKER_PACKAGES=${DOCKER_PACKAGES:-build-essential libtool autotools-dev automake pkg-config bsdmainutils curl ca-certificates ccache python3 rsync git procps}