PackageModelTests: fix warnings (#7421)

Resolve warnings in `Tests/PackageModelTests/MinimumDeploymentTargetTests.swift` .

### Motivation:

3 warnings were displayed indicating that a deprecated function is being
used and suggesting the use of an alternative function.

### Modifications:

Following the suggestion, the warning was resolved.

### Result:

The following warnings have been resolved:

> `init(arguments:environment:exitStatus:output:stderrOutput:)` is
deprecated: use
`init(arguments:environmentBlock:exitStatus:output:stderrOutput:)`

Tests/PackageModelTests/MinimumDeploymentTargetTests.swift

@@ -22,7 +22,7 @@ class MinimumDeploymentTargetTests: XCTestCase {
  try XCTSkipIf(true, "test is only supported on macOS")
  #endif
  let result = ProcessResult(arguments: [],
- environment: [:],
+ environmentBlock: [:],
  exitStatus: .terminated(code: 0),
  output: "".asResult,
  stderrOutput: "xcodebuild: error: SDK \"macosx\" cannot be located.".asResult)
@@ -36,7 +36,7 @@ class MinimumDeploymentTargetTests: XCTestCase {
  try XCTSkipIf(true, "test is only supported on macOS")
  #endif
  let result = ProcessResult(arguments: [],
- environment: [:],
+ environmentBlock: [:],
  exitStatus: .terminated(code: 0),
  output: "some string".asResult,
  stderrOutput: "".asResult)
@@ -50,7 +50,7 @@ class MinimumDeploymentTargetTests: XCTestCase {
  try XCTSkipIf(true, "test is only supported on macOS")
  #endif
  let result = ProcessResult(arguments: [],
- environment: [:],
+ environmentBlock: [:],
  exitStatus: .terminated(code: 0),
  output: .failure(DummyError()),
  stderrOutput: "".asResult)