Try passing the _real_ windows dir to innoinstaller, then using the linux-like directory for mv.

CI/before_script.msvc.sh

@@ -417,15 +417,16 @@ fi
 		# Boost's installer is still based on ms-dos API that doesn't support larger than 260 char path names
 		# We work around this by installing to root of the current working drive and then move it to our deps
 		# get the current working drive's root, we'll install to that temporarily
-		CWD_DRIVE_ROOT=$(echo "$(powershell -command '(get-location).Drive.Root')" | sed "s,\\\\,/,g") 
+		CWD_DRIVE_ROOT="$(powershell -command '(get-location).Drive.Root')Boost_temp"
+		CWD_DRIVE_ROOT_BASH=$(echo "$CWD_DRIVE_ROOT" | sed "s,\\\\,/,g" | sed "s,\(.\):,/\\1,")
 		
 		if [ -d Boost ] && grep "BOOST_VERSION 106700" Boost/boost/version.hpp > /dev/null; then
 			printf "Exists. "
 		elif [ -z $SKIP_EXTRACT ]; then
 			rm -rf Boost
 			[ -n "$CI" ] && CI_EXTRA_INNO_OPTIONS="//SUPPRESSMSGBOXES //LOG='boost_install.log'"
-			"${DEPS}/boost-1.67.0-msvc${MSVC_YEAR}-win${BITS}.exe" //DIR="${CWD_DRIVE_ROOT}Boost_temp" //VERYSILENT //NORESTART ${CI_EXTRA_INNO_OPTIONS}
-			mv "${CWD_DRIVE_ROOT}Boost_temp" "${BOOST_SDK}"
+			"${DEPS}/boost-1.67.0-msvc${MSVC_YEAR}-win${BITS}.exe" //DIR="${CWD_DRIVE_ROOT}" //VERYSILENT //NORESTART ${CI_EXTRA_INNO_OPTIONS}
+			mv "${CWD_DRIVE_ROOT_BASH}" "${BOOST_SDK}"
 		fi
 		add_cmake_opts -DBOOST_ROOT="$BOOST_SDK" \
 			-DBOOST_LIBRARYDIR="${BOOST_SDK}/lib${BITS}-msvc-${MSVC_VER}"