bst shell work directory no longer at /buildstream-build on buildstream 2.6.0

Reproduce the regression by running the shown bst shell utils/generate-cve-report.bst commands with BuildStream 2.5.0 and 2.6.0, then test the bst shell path with --mount cve /buildstream-build. Trace the shell entry point and verify that the mounted files remain accessible at the expected working directory without breaking the 2.6.0 behavior.

In buildstream 2.5.0 and before, bst shell <element.bst> will start the shell in /buildstream-build, e.g.

src # bst --version
2.5.0
src # bst shell utils/generate-cve-report.bst
[--:--:--][        ][    main:core activity                 ] START   Loading elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Loading elements
[--:--:--][        ][    main:core activity                 ] START   Resolving elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Resolving elements
[--:--:--][        ][    main:core activity                 ] START   Initializing remote caches
[00:00:01][        ][    main:core activity                 ] SUCCESS Initializing remote caches
[--:--:--][        ][    main:core activity                 ] START   Query cache
[00:00:01][        ][    main:core activity                 ] SUCCESS Query cache
[--:--:--][6deaf86b][    main:utils/generate-cve-report.bst ] START   Staging dependencies
[00:00:00][6deaf86b][    main:utils/generate-cve-report.bst ] SUCCESS Staging dependencies
[--:--:--][6deaf86b][    main:utils/generate-cve-report.bst ] START   Integrating sandbox
[00:00:00][6deaf86b][    main:utils/generate-cve-report.bst ] SUCCESS Integrating sandbox
[--:--:--][6deaf86b][    main:utils/generate-cve-report.bst ] STATUS  Running command

    sh -i

<:/buildstream-build]$ pwd
/buildstream-build

In buildstream 2.6.0, the work directory is instead /

src # bst --version
2.6.0
src # bst shell utils/generate-cve-report.bst
[--:--:--][        ][    main:core activity                 ] START   Loading elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Loading elements
[--:--:--][        ][    main:core activity                 ] START   Resolving elements
[00:00:00][        ][    main:core activity                 ] SUCCESS Resolving elements
[--:--:--][        ][    main:core activity                 ] START   Initializing remote caches
[00:00:01][        ][    main:core activity                 ] SUCCESS Initializing remote caches
[--:--:--][        ][    main:core activity                 ] START   Query cache
[00:00:00][        ][    main:core activity                 ] SUCCESS Query cache
[--:--:--][6deaf86b][    main:utils/generate-cve-report.bst ] START   Staging dependencies
[00:00:00][6deaf86b][    main:utils/generate-cve-report.bst ] SUCCESS Staging dependencies
[--:--:--][6deaf86b][    main:utils/generate-cve-report.bst ] START   Integrating sandbox
[00:00:00][6deaf86b][    main:utils/generate-cve-report.bst ] SUCCESS Integrating sandbox
[--:--:--][6deaf86b][    main:utils/generate-cve-report.bst ] STATUS  Running command

    sh -i

<te-cve-report.bst:/]$ pwd
/
<te-cve-report.bst:/]$ ls
bin  dev  etc  lib  lib64  proc  sbin  tmp  usr  var

We have issue when some files are mounted to /buildstream-build and are accessed by relative path with bst shell <element> --mount cve /buildstream-build -- <script-accessing-mounted-files>