Code signature invalid for arm64 Mambaforge installer

- [x] I read [the conda-forge documentation](https://conda-forge.org/docs/user/introduction.html#how-can-i-install-packages-from-conda-forge) and could not find the solution for my problem there.

Issue: I downloaded https://github.com/conda-forge/miniforge/releases/latest/download/Miniforge3-MacOSX-x86_64.sh yesterday. The installation seemed to go fine, except for initializing zsh. After putting `~/mambaforge/bin` on the path, I see:
```
% mamba
zsh: killed mamba
% conda
zsh: killed conda
```

Looking at `Console > Crash Reports`, the reason seems to be the code signing having an issue:
```
Exception Type: EXC_BAD_ACCESS (Code Signature Invalid)
Exception Codes: 0x0000000000000032, 0x00000001040cc000
Exception Note: EXC_CORPSE_NOTIFY

Termination Reason: Namespace CODESIGNING, Code 0x2
```

Full details:

```
Process: python3.9 [19158]
Path: /Users/USER/*/python3.9
Identifier: python3.9
Version: ???
Code Type: ARM-64 (Native)
Parent Process: zsh [19151]
Responsible: Terminal [19149]
User ID: 501

Date/Time: 2021-07-22 23:41:27.736 +0200
OS Version: macOS 11.4 (20F71)
Report Version: 12
Anonymous UUID: 2E0F70D6-E42E-68B1-CEB0-EFC8E97D90E0

Sleep/Wake UUID: F8BF3778-104E-4E48-A4F3-CA9B14154147

Time Awake Since Boot: 94000 seconds
Time Since Wake: 9400 seconds

System Integrity Protection: enabled

Crashed Thread: Unknown

Exception Type: EXC_BAD_ACCESS (Code Signature Invalid)
Exception Codes: 0x0000000000000032, 0x00000001040cc000
Exception Note: EXC_CORPSE_NOTIFY

Termination Reason: Namespace CODESIGNING, Code 0x2

kernel messages:

VM Regions Near 0x1040cc000:
--> mapped file 1040cc000-1043d4000 [ 3104K] r-x/r-x SM=COW Object_id=82618143
mapped file 1043d4000-104414000 [ 256K] rw-/rw- SM=COW Object_id=82618143

Backtrace not available

Unknown thread crashed with ARM Thread State (64-bit):
x0: 0x00000001040cc000 x1: 0x000000016bd338f8 x2: 0x000000016bd338a8 x3: 0x000000010450df03
x4: 0x0000000000000070 x5: 0x0000000000000073 x6: 0x000000016bd33650 x7: 0x0000000000000630
x8: 0x000000016bd338d8 x9: 0x000000016bd338b8 x10: 0x0000000104532a78 x11: 0x0000000000000003
x12: 0x0000000000000003 x13: 0x00000000fffffffc x14: 0x0000000000000000 x15: 0x0000000000000000
x16: 0x00000001044e2084 x17: 0x6ae100016bd338a8 x18: 0x0000000000000000 x19: 0x000000016bd338f8
x20: 0x00000001040cc000 x21: 0x000000016bd338a8 x22: 0x00000001040cc000 x23: 0x000000016bd33a60
x24: 0x000000016bd33b08 x25: 0x000000016bd33990 x26: 0x0000000000000000 x27: 0x0000000000000000
x28: 0x000000016bd33a30 fp: 0x000000016bd33890 lr: 0x0a7f0001044e204c
sp: 0x000000016bd33810 pc: 0x00000001044e4470 cpsr: 0x80000000
far: 0x00000001040cc000 esr: 0x92000007

Binary images description not available

External Modification Summary:
Calls made by other processes targeting this process:
task_for_pid: 0
thread_create: 0
thread_set_state: 0
Calls made by this process:
task_for_pid: 0
thread_create: 0
thread_set_state: 0
Calls made by all processes on this machine:
task_for_pid: 0
thread_create: 0
thread_set_state: 0

VM Region Summary:
Writable regions: Total=16.3M written=0K(0%) resident=0K(0%) swapped_out=0K(0%) unallocated=16.3M(100%)

VIRTUAL REGION
REGION TYPE SIZE COUNT (non-coalesced)
=========== ======= =======
STACK GUARD 16K 1
Stack 16.0M 1
VM_ALLOCATE 1.0G 2
VM_ALLOCATE (reserved) 144K 2 reserved VM address space (unallocated)
mapped file 4592K 7
=========== ======= =======
TOTAL 1.0G 13
TOTAL, minus reserved VM space 1.0G 13
```

Searching the docs (https://conda-forge.org/docs/orga/minutes/2020-11-11.html?highlight=signature) shows:
_Anaconda may add signing to conda-build at some point in expectation that apple will require real certs and not simply an ad-hoc signature_

The disclaimer on the download page says the `arm64` builds are still experimental, but this may be a recent change since I assume they were able to at least run `conda` and `mamba` before.