chore(dev/release): Fix Windows verification under conda (#1684)

This changes two things to make release verification run on my system:

- Adds ` | Resolve-Path` to three variables to make them absolute paths.
Without this, cpp_build.ps1 was running CMake and pointing at the wrong
path because it saves a relative path, cds into another directory, and
then tries to use the relative path.
- Adds `m2w64-gcc` inline to the `mamba create` call to make sure the Go
has access to gcc for building. I was a little surprised the existing
`compilers` package doesn't install gcc so I'm not sure if something
might have changed upstream.

Fixes #1683

dev/release/verify-release-candidate.ps1

@@ -92,6 +92,8 @@ if ($SourceKind -eq "local") {
 } else {
     $ArrowSourceDir = Join-Path $ArrowTempDir $DistName
     New-Item -ItemType Directory -Path $ArrowSourceDir -Force
+    # Convert to an absolute now that it should exist
+    $ArrowSourceDir = $ArrowSourceDir | Resolve-Path | % { $_.Path }
 
     Download-Dist-File "$($DistName).tar.gz"
     Download-Dist-File "$($DistName).tar.gz.sha512"
@@ -115,7 +117,8 @@ Show-Header "Create Conda Environment"
 mamba create -c conda-forge -f -y -p $(Join-Path $ArrowTempDir conda-env) `
   --file $(Join-Path $ArrowSourceDir ci\conda_env_cpp.txt) `
   --file $(Join-Path $ArrowSourceDir ci\conda_env_python.txt) `
-  go
+  go `
+  m2w64-gcc
 
 Invoke-Expression $(conda shell.powershell hook | Out-String)
 conda activate $(Join-Path $ArrowTempDir conda-env)