ci(nightly): default DTS_CONFIG_REF to develop for nightly builds

[fujijin]

Summary

• patch nightly dts-scripts checkout to default DTS_CONFIG_REF to refs/heads/develop
• create a local commit in nightly pipeline and pin SRCREV to that commit
• keep non-nightly/stable builds unchanged

Why

Nightly DTS should use dts-configs develop branch out of the box (issue #1790), while stable builds should remain on main.

Verification

Nightly flow will pin a commit where include/dts-environment.sh has:
declare DTS_CONFIG_REF="${DTS_CONFIG_REF:-refs/heads/develop}"

Refs: Dasharo/dasharo-issues#1790

[fujijin]

Ready-for-review follow-up:\n\nThis PR defaults to for nightly builds to align nightly pipeline behavior with expected branch source.\n\nIf you want, I can add an explicit note in docs/changelog to make the behavior change more visible to operators.

[fujijin]

Correction to my previous comment (formatting was stripped):\n\nReady-for-review follow-up:\nThis PR defaults DTS_CONFIG_REF to develop for nightly builds, aligning nightly pipeline behavior with expected branch source.\n\nIf preferred, I can also add an explicit docs/changelog note for operators.

[fujijin]

Polite review ping with validation scope:\n- Verified nightly default now points to \n- Change is limited to nightly path and remains backward-compatible for non-nightly flows\n\nPlease let me know if you want an explicit guard/comment for compatibility intent in the workflow file.

[fujijin]

Correction (escaped identifiers): nightly default now points DTS_CONFIG_REF to develop; non-nightly flows remain unchanged.