Fix size test (stripe#3490)

## Summary
* Only set LD_GENERATE_MAP_FILE in the app target — if we add it to the
xcodebuild command, it will also generate a file at the same path for
each framework.
* Set LD_GENERATE_MAP_FILE in the SPMTest.xcodeproj project, and
regenerate the patch.
* Use Xcode 15.3
* Always checkout from the `measure_branch`: Because of how Bitrise
checks out branches in recent stacks, `git rev-parse --abbrev-ref HEAD`
was just returning `HEAD`, which was causing us to then re-checkout
`HEAD` after checking out `master`, which triggered it to use an *old*
version of the Xcodeproj patch. This was confusing and caused it to not
pick up changes from this PR.

## Motivation
Size_test was broken in Xcode 15

## Testing
CI

Gemfile

@@ -9,12 +9,8 @@ gem "rubyzip"
 gem "octokit", "~> 4.25.1"
 gem "mime-types"
 gem "CFPropertyList"
-gem "sqlite3", "~> 1.4.4"
 gem "terminal-table"
 
-gem "jazzy", "0.14.1"
-gem "mustache", "~> 1.1.1"
-
 gem "xcode-install", "~> 2.6"
 gem "plist"
 gem "net-ssh"