build: Make merge target optional

[AbhiPrasad]

Make merge_target not merge to master by default so we can work off the v7 branch and cut betas off it.

Resolves https://getsentry.atlassian.net/browse/WEB-654

[github-actions]

size-limit report

Path	Base Size (4eadcaf)	Current Size	Change
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.49 KB	19.49 KB	+0.01% 🔺
@sentry/browser - ES5 CDN Bundle (minified)	62.17 KB	62.17 KB	0%
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.11 KB	18.12 KB	+0.03% 🔺
@sentry/browser - ES6 CDN Bundle (minified)	55.5 KB	55.5 KB	0%
@sentry/browser - Webpack (gzipped + minified)	22.6 KB	22.6 KB	0%
@sentry/browser - Webpack (minified)	79.21 KB	79.21 KB	0%
@sentry/react - Webpack (gzipped + minified)	22.63 KB	22.63 KB	0%
@sentry/nextjs Client - Webpack (gzipped + minified)	47.6 KB	47.6 KB	0%
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.36 KB	25.36 KB	+0.01% 🔺
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.72 KB	23.72 KB	+0.01% 🔺