Fix build warnings in the workflow for iOS

IMHO, it would be better to use `$(xcrun -f swift)` ("find" mode)
instead of `xcrun swift` ("run" mode).

`xcrun swift` is fine with packages that do not contain
`executableTarget`, but it causes the following errors with packages
that contain `executableTarget`:

```
clang: warning: using sysroot for 'MacOSX' but targeting 'iPhone'
[-Wincompatible-sysroot]
```

To prevent this, we can use `$(xcrun -f swift)` instead of `xcrun
swift`.

Related discussion:
https://forums.swift.org/t/getting-incompatible-sysroot-warning-when-compiling-for-ios-with-sdk-and-target-set/80584

Author: kkebo

.github/workflows/pull_request.yml

@@ -90,7 +90,7 @@ jobs:
       enable_ios_checks: true
       ios_pre_build_command: |
         pwd
-        xcrun swift package init --type library
+        "$(xcrun -f swift)" package init --type library
 
   soundness:
     name: Soundness

.github/workflows/swift_package_test.yml

@@ -110,7 +110,7 @@ on:
       ios_build_command:
         type: string
         description: "macOS command to build the package for iOS"
-        default: "xcrun swift build --build-tests --sdk \"$(xcrun --sdk iphoneos --show-sdk-path)\" --triple arm64-apple-ios"
+        default: "\"$(xcrun -f swift)\" build --build-tests --sdk \"$(xcrun --sdk iphoneos --show-sdk-path)\" --triple arm64-apple-ios"
       linux_build_command:
         type: string
         description: "Linux command to build and test the package"
@@ -255,7 +255,8 @@ jobs:
       - name: Select Xcode
         run: echo "DEVELOPER_DIR=/Applications/Xcode_${{ matrix.xcode_version }}.app" >> $GITHUB_ENV
       - name: Swift version
-        run: xcrun swift --version
+        run: |
+          "$(xcrun -f swift)" --version
       - name: Pre-build
         run: ${{ inputs.ios_pre_build_command }}
       - name: Build