Alire.Environment: change _PREFIX to _ALIRE_PREFIX (#958)

* alire.toml: bump gnatcoll-slim commit to match git submodule

* Alire.Environment: change _PREFIX to _ALIRE_PREFIX

With just _PREFIX there is a high risk of conflicts with legitimate use
of environment variables.

src/alire/alire-environment.adb

@@ -213,7 +213,7 @@ package body Alire.Environment is
       end loop;
 
       --  Set the crate PREFIX location for access to resources
-      This.Set (AAA.Strings.To_Upper_Case (+Rel.Name) & "_PREFIX",
+      This.Set (AAA.Strings.To_Upper_Case (+Rel.Name) & "_ALIRE_PREFIX",
                 Release_Base,
                 "Crate prefix for resources location");
    end Load;

testsuite/tests/printenv/basic/test.py

@@ -36,9 +36,9 @@ def make_path(list):
              '.*'
              'export GPR_PROJECT_PATH="' + expected_gpr_path + '"\n'
              '.*'
-             'export HELLO_PREFIX="' + expected_hello_path + '"\n'
+             'export HELLO_ALIRE_PREFIX="' + expected_hello_path + '"\n'
              '.*'
-             'export LIBHELLO_PREFIX="' + expected_libhello_path + '"\n'
+             'export LIBHELLO_ALIRE_PREFIX="' + expected_libhello_path + '"\n'
              '.*'
              'export TEST_GPR_EXTERNAL="gpr_ext_B"\n'
              '.*',