authctl: Fix int overflow in test on 32-bit architectures

[adombeck]

On 32-bit architectures, math.MaxInt32 + 1 is larger than the int type, so it causes the test to fail with

cmd/authctl/group/set-gid_test.go:52:65: cannot use math.MaxInt32 + 1 (untyped int constant 2147483648) as int value in argument to strconv.Itoa (overflows)

The fix is to use an int64 instead.

refs: https://bugs.launchpad.net/ubuntu/+source/authd/+bug/2148363

UDENG-9791

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.10%. Comparing base (15c4664) to head (2cdb432).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1454      +/-   ##
==========================================
+ Coverage   87.05%   87.10%   +0.04%     
==========================================
  Files          93       93              
  Lines        6367     6367              
  Branches      111      111              
==========================================
+ Hits         5543     5546       +3     
+ Misses        768      765       -3     
  Partials       56       56              

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