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

In buildstream 2.5.0 and before, `bst shell ` 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

--mount cve /buildstream-build -- `

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.

Written by the indexing model from the issue text.