builder-flatpak-utils: Fix const correctness warnings

[bbhtt]

Glibc 2.43 implements C23 const-preserving string-search functions 1

This fixes the build with Glibc 2.43 and GCC 15.1 (which defaults to C23).

../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.