fix(core): Add dsn to span envelope header

[mydea]

This was forgotten/got lost while we migrated this from v7 to v8. Now, adding the dsn again when the tunnel is enabled.

Fixes #12094

[Lms24]

Good catch!

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	21.72 KB (0%)
@sentry/browser (incl. Tracing)	32.88 KB (+0.06% 🔺)
@sentry/browser (incl. Tracing, Replay)	68.23 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.61 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas)	72.27 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback)	84.27 KB (+0.02% 🔺)
@sentry/browser (incl. Feedback)	37.68 KB (0%)
@sentry/browser (incl. sendFeedback)	26.29 KB (0%)
@sentry/browser (incl. FeedbackAsync)	30.66 KB (0%)
@sentry/react	24.41 KB (0%)
@sentry/react (incl. Tracing)	35.87 KB (+0.04% 🔺)
@sentry/vue	25.66 KB (+0.05% 🔺)
@sentry/vue (incl. Tracing)	34.69 KB (+0.05% 🔺)
@sentry/svelte	21.86 KB (0%)
CDN Bundle	24.26 KB (0%)
CDN Bundle (incl. Tracing)	34.31 KB (+0.06% 🔺)
CDN Bundle (incl. Tracing, Replay)	68 KB (+0.03% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback)	72.97 KB (+0.03% 🔺)
CDN Bundle - uncompressed	71.33 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	101.74 KB (+0.07% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	211.36 KB (+0.04% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	223.66 KB (+0.03% 🔺)
@sentry/nextjs (client)	35.09 KB (+0.05% 🔺)
@sentry/sveltekit (client)	33.46 KB (+0.05% 🔺)
@sentry/node	141.32 KB (0%)
@sentry/aws-serverless	128.08 KB (0%)