Test patch:
$ cat files/gnupg-2.1.21-force-homedir-socket.patch
--<-------------------------------------------------------------------->-- 
diff --git a/common/homedir.c b/common/homedir.c
index fce6d44be..79f5042d1 100644
--- a/common/homedir.c
+++ b/common/homedir.c
@@ -488,7 +488,7 @@ _gnupg_socketdir_internal (int skip_checks, unsigned *r_info)
       if (!stat (prefix, &sb) && S_ISDIR(sb.st_mode))
         break;
     }
-  if (!bases[i])
+  if (!bases[i] || (getenv("GNUPG_HOMEDIR_SOCKET") && strcmp(getenv("GNUPG_HOMEDIR_SOCKET"), "force") == 0))
     {
       *r_info |= 2; /* No /run/user directory.  */
       goto leave;
--<-------------------------------------------------------------------->--

Test script:
--<-------------------------------------------------------------------->--
#!/bin/bash
#setup directories
rm -rf /tmp/gnupg
mkdir -p /tmp/gnupg/{a,b}
rm -rf /run/user/$UID/gnupg

#first normal run, we should get no socket in homedir as it is in /run/user/$UID
export GNUPGHOME=/tmp/gnupg/a
gpg --recv-key 94CBAFDD30345109561835AA0B7F8B60E3EDFAE3
gpg -K 
ls -la /tmp/gnupg/a
ls -la /run/user/$UID/gnupg
rm -rf /run/user/$UID/gnupg

#Now we should force socket in homedir instead
export GNUPGHOME=/tmp/gnupg/b
export GNUPG_HOMEDIR_SOCKET="force"
gpg --recv-key 94CBAFDD30345109561835AA0B7F8B60E3EDFAE3
gpg -K 
ls -la /tmp/gnupg/b
ls -la /run/user/$UID/gnupg
rm -rf /run/user/$UID/gnupg


--<-------------------------------------------------------------------->--

kristianf@ares ~ $ ./gnupg-test.sh
gpg: WARNING: unsafe permissions on homedir '/tmp/gnupg/a'
gpg: keybox '/tmp/gnupg/a/pubring.kbx' created
gpg: /tmp/gnupg/a/trustdb.gpg: trustdb created
gpg: key 0B7F8B60E3EDFAE3: public key "Kristian Fiskerstrand <kristian.fiskerstrand@sumptuouscapital.com>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1
gpg: WARNING: unsafe permissions on homedir '/tmp/gnupg/a'
total 384
drwxr-xr-x 4 kristianf kristianf    140 Jul  9 00:41 .
drwxr-xr-x 4 kristianf kristianf     80 Jul  9 00:41 ..
drwx------ 2 kristianf kristianf     60 Jul  9 00:41 crls.d
drwx------ 2 kristianf kristianf     40 Jul  9 00:41 private-keys-v1.d
-rw-r--r-- 1 kristianf kristianf 381005 Jul  9 00:41 pubring.kbx
-rw------- 1 kristianf kristianf     32 Jul  9 00:41 pubring.kbx~
-rw------- 1 kristianf kristianf   1200 Jul  9 00:41 trustdb.gpg
total 0
drwx------ 3 kristianf kristianf  60 Jul  9 00:41 .
drwx------ 8 kristianf root      160 Jul  9 00:41 ..
drwx------ 2 kristianf kristianf 140 Jul  9 00:41 d.qfmawbqooe59swes3oaranoy
gpg: WARNING: unsafe permissions on homedir '/tmp/gnupg/b'
gpg: keybox '/tmp/gnupg/b/pubring.kbx' created
gpg: /tmp/gnupg/b/trustdb.gpg: trustdb created
gpg: key 0B7F8B60E3EDFAE3: public key "Kristian Fiskerstrand <kristian.fiskerstrand@sumptuouscapital.com>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1
gpg: WARNING: unsafe permissions on homedir '/tmp/gnupg/b'
total 384
drwxr-xr-x 4 kristianf kristianf    240 Jul  9 00:41 .
drwxr-xr-x 4 kristianf kristianf     80 Jul  9 00:41 ..
drwx------ 2 kristianf kristianf     60 Jul  9 00:41 crls.d
drwx------ 2 kristianf kristianf     40 Jul  9 00:41 private-keys-v1.d
-rw-r--r-- 1 kristianf kristianf 381005 Jul  9 00:41 pubring.kbx
-rw------- 1 kristianf kristianf     32 Jul  9 00:41 pubring.kbx~
srwx------ 1 kristianf kristianf      0 Jul  9 00:41 S.dirmngr
srwx------ 1 kristianf kristianf      0 Jul  9 00:41 S.gpg-agent
srwx------ 1 kristianf kristianf      0 Jul  9 00:41 S.gpg-agent.browser
srwx------ 1 kristianf kristianf      0 Jul  9 00:41 S.gpg-agent.extra
srwx------ 1 kristianf kristianf      0 Jul  9 00:41 S.gpg-agent.ssh
-rw------- 1 kristianf kristianf   1200 Jul  9 00:41 trustdb.gpg
ls: cannot access '/run/user/1000/gnupg': No such file or directory