ci: Use ubuntu 26.04 container

[bbhtt]

Needed for new dependencies like appstream, GitHub is yet to make actions-images available for 26.04

[bbhtt]

../src/builder-flatpak-utils.c: In function ‘flatpak_path_match_prefix’:
../src/builder-flatpak-utils.c:188:27: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  188 |               char *tmp = strchr (string, '/');
      |                           ^~~~~~
../src/builder-flatpak-utils.c: In function ‘get_xdg_user_dir_from_string’:
../src/builder-flatpak-utils.c:1641:9: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 1641 |   slash = strchr (filesystem, '/');
      |         ^
../src/builder-flatpak-utils.c: In function ‘option_add_generic_policy_cb’:
../src/builder-flatpak-utils.c:2094:5: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 2094 |   t = strchr (value, '=');
      |     ^
../src/builder-flatpak-utils.c: In function ‘option_remove_generic_policy_cb’:
../src/builder-flatpak-utils.c:2122:5: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 2122 |   t = strchr (value, '=');
      |   

/usr/include/ostree-1/ostree-content-writer.h:28:23: warning: redefinition of typedef 'OstreeContentWriter' is a C11 feature [-Wtypedef-redefinition]
   28 | G_DECLARE_FINAL_TYPE (OstreeContentWriter, ostree_content_writer, OSTREE, CONTENT_WRITER,
      |                       ^
/usr/include/ostree-1/ostree-types.h:39:37: note: previous definition is here
   39 | typedef struct _OstreeContentWriter OstreeContentWriter;
      |                                     ^
../src/builder-flatpak-utils.c:188:21: error: initializing 'char *' with an expression of type 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  188 |               char *tmp = strchr (string, '/');
      |                     ^     ~~~~~~~~~~~~~~~~~~~~
../src/builder-flatpak-utils.c:1641:9: error: assigning to 'char *' from 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
 1641 |   slash = strchr (filesystem, '/');
      |         ^ ~~~~~~~~~~~~~~~~~~~~~~~~
../src/builder-flatpak-utils.c:2094:5: error: assigning to 'char *' from 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
 2094 |   t = strchr (value, '=');
      |     ^ ~~~~~~~~~~~~~~~~~~~
../src/builder-flatpak-utils.c:2122:5: error: assigning to 'char *' from 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
 2122 |   t = strchr (value, '=');
      |     ^ ~~~~~~~~~~~~~~~~~~~
1 warning and 4 errors generated.

edit. split to #739