Expand variables in the sandbox section of project.conf
Description
As described in #1303 (closed) setting sandbox:build-arch
to a variable used to cause a Unresolved variable
error
venvbst2 will localhost ~ projects buildstream keytest willsalmon/sandboxarch ✎ $ bst -o target_arch aarch64 build basic-import.bst
[17:17:31][--:--:--][ ][ main:core activity ] START Build
[17:17:31][--:--:--][ ][ main:core activity ] START Loading elements
[17:17:31][00:00:00][ ][ main:core activity ] SUCCESS Loading elements
[17:17:31][--:--:--][ ][ main:core activity ] START Resolving elements
print /home/will/projects/buildstream/keytest/project.conf [line 23 column 2]: {'build-arch': '%{target_arch}'}
[17:17:31][00:00:00][ ][ main:core activity ] FAILURE Resolving elements
[17:17:31][00:00:00][ ][ main:core activity ] FAILURE Build
/home/will/projects/buildstream/keytest/project.conf [line 23 column 14]: Unresolved variable: target_arch
This MR resolves #1303 (closed) and adds a test.