EES-2236 Fix missing zip file permissions in Unix environments

Currently, .NET doesn't automatically add Unix file permissions to our
zip's file entries. This means that all of the files cannot be opened
without setting the permissions manually in the OS.

To fix this, we manually add the Unix file permissions ourselves by
converting a permission string to an octal literal on the zip entry.

I believe that in .NET 6, this should be done automatically for us due
to the following change: dotnet/runtime#55531

src/GovUk.Education.ExploreEducationStatistics.Common/Extensions/ZipArchiveExtensions.cs

@@ -0,0 +1,24 @@
+#nullable enable
+using System;
+using System.IO.Compression;
+
+namespace GovUk.Education.ExploreEducationStatistics.Common.Extensions
+{
+    public static class ZipArchiveExtensions
+    {
+        /// <summary>
+        /// Adds UNIX file permissions to the zip entry.
+        /// </summary>
+        /// <remarks>
+        /// We need this due to .NET not adding these permissions for us.
+        /// See: https://github.com/dotnet/runtime/issues/1548
+        /// This should be fixed in .NET 6 as part of the following PR:
+        /// https://github.com/dotnet/runtime/pull/55531
+        /// </remarks>
+        public static ZipArchiveEntry SetUnixPermissions(this ZipArchiveEntry entry, string permissions)
+        {
+            entry.ExternalAttributes |= Convert.ToInt32(permissions, 8) << 16;
+            return entry;
+        }
+    }
+}

src/GovUk.Education.ExploreEducationStatistics.Content.Services/ReleaseFileService.cs

@@ -125,7 +125,10 @@ private async Task DoZipFilesToStream(
                     continue;
                 }
 
-                var entry = archive.CreateEntry(releaseFile.File.ZipFileEntryName());
+                var entry = archive
+                    .CreateEntry(releaseFile.File.ZipFileEntryName())
+                    .SetUnixPermissions("664");
+
                 await using var entryStream = entry.Open();
 
                 await _blobStorageService.DownloadToStream(
@@ -144,7 +147,10 @@ await _blobStorageService.DownloadToStream(
 
             if (subjectIds.Any())
             {
-                var entry = archive.CreateEntry(FileType.DataGuidance.GetEnumLabel() + "/data-guidance.txt");
+                var entry = archive
+                    .CreateEntry(FileType.DataGuidance.GetEnumLabel() + "/data-guidance.txt")
+                    .SetUnixPermissions("664");
+
                 await using var entryStream = entry.Open();
 
                 await _dataGuidanceFileWriter.WriteToStream(entryStream, release, subjectIds);