Fails to create build directory for elements with workspaces

This is a recent regression. I ran bst build core.bst and it ended with:

[00:00:00][????????][build:core/glib-networking.bst      ] FAILURE Running build-commands
[00:00:00][????????][build:core/glib-networking.bst      ] FAILURE Running build-commands
[00:00:26][????????][build:core/glib-networking.bst      ] FAILURE Command 'ninja -j ${NINJAJOBS} -C _builddir' failed with exitcode 1

    Sandbox directory: /home/mcatanzaro/.cache/buildstream/build/core-glib-networking-13pb8fda

    Printing the last 20 lines from log file:
    /home/mcatanzaro/.cache/buildstream/logs/gnome/core-glib-networking/????????-build.11243.log
    ======================================================================
        glib-compile-schemas /usr/share/glib-2.0/schemas
    Warning: Schema “org.gnome.system.locale” has path “/system/locale/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
    Warning: Schema “org.gnome.system.proxy” has path “/system/proxy/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
    Warning: Schema “org.gnome.system.proxy.http” has path “/system/proxy/http/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
    Warning: Schema “org.gnome.system.proxy.https” has path “/system/proxy/https/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
    Warning: Schema “org.gnome.system.proxy.ftp” has path “/system/proxy/ftp/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
    Warning: Schema “org.gnome.system.proxy.socks” has path “/system/proxy/socks/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
    [00:00:18] SUCCESS [core/glib-networking.bst]: Integrating sandbox
    [--:--:--] START   [core/glib-networking.bst]: Staging sources
    [00:00:00] SUCCESS [core/glib-networking.bst]: Staging sources
    [--:--:--] START   [core/glib-networking.bst]: Running build-commands
    [--:--:--] START   [core/glib-networking.bst]: Running build-commands
    [--:--:--] STATUS  [core/glib-networking.bst]: Running build-commands

        ninja -j ${NINJAJOBS} -C _builddir
    ninja: Entering directory `_builddir'
    ninja: fatal: chdir to '_builddir' - No such file or directory
    [00:00:00] FAILURE [core/glib-networking.bst]: Running build-commands
    [00:00:00] FAILURE [core/glib-networking.bst]: Running build-commands
    [00:00:26] FAILURE [core/glib-networking.bst]: Command 'ninja -j ${NINJAJOBS} -C _builddir' failed with exitcode 1
    ======================================================================

BuildStream should create the build directory, instead of failing because it doesn't exist.