fix: SPM manifest compilation on Swift 6.1 (#7819)

`String(cString:encoding:)` is a Foundation extension on `String`
and is not available in the SwiftPM manifest context, where only
`PackageDescription` is imported. On Swift 6.1 / Xcode 16+,
`swift package resolve` now fails with:

    error: extra argument 'encoding' in call

Use the stdlib `String(cString:)` initializer, which already
decodes as UTF-8 and matches the original intent.

---------

Co-authored-by: SeungHyun Hong <whi7ehyun@gmail.com>

Author: itaybre

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Fixes
+
+- Fix `Package@swift-6.1.swift` SPM manifest compilation on Swift 6.1 / Xcode 26 (#7800, #7819)
+
 ## 8.58.1
 
 ### Fixes

Package@swift-6.1.swift

@@ -69,7 +69,7 @@ var targets: [Target] = [
 ]
 
 let env = getenv("EXPERIMENTAL_SPM_BUILDS")
-if let env = env, String(cString: env, encoding: .utf8) == "1" {
+if let env = env, String(cString: env) == "1" {
     products.append(.library(name: "SentrySPM", type: .dynamic, targets: ["SentryObjc"]))
     targets.append(contentsOf: [
         // At least one source file is required, therefore we use a dummy class to satisfy the SPM build system