Merge bitcoin/bitcoin#29953: doc: Bash is needed in gen_id and is not installed on FreeBSD by default

9381052 doc: Bash is needed in gen_id and is not installed on FreeBSD by default (Hennadii Stepanov)

Pull request description:

  On FreeBSD 14.0, in the `depends` directory:

  - without `bash`:
  ```
  $ gmake print-bdb_build_id_long
  env: bash: No such file or directory
  env: bash: No such file or directory
  bdb_build_id_long=bdb-4.8.30-4b0c6f8e95251b9c6731844fc34111c04b75fd9f15c671d6e34f2a4d014ec1be-release
  $ gmake print-final_build_id
  env: bash: No such file or directory
  env: bash: No such file or directory
  final_build_id=722b2d3e264
  ```

  - with `bash`:
  ```
  $ gmake print-bdb_build_id_long
  bdb_build_id_long=bdb-4.8.30-4b0c6f8e95251b9c6731844fc34111c04b75fd9f15c671d6e34f2a4d014ec1be-release  1ed47cefe468014c79dedb275cf921f44ab28d91dd56bf94712409b81326d765
  $ gmake print-final_build_id
  final_build_id=7b4f9aaa683
  ```

ACKs for top commit:
  vasild:
    ACK 9381052
  kristapsk:
    ACK 9381052
  alfonsoromanz:
    ACK 9381052

Tree-SHA512: da3f3469ac416518180194f09fb054fb352a2793848fb9a7982439de08244ff6149a7f449ad21fcdf0e9bd79b6949a91751f9cc35833953d2b6a35cea5c6ae21

Author: fanquake

depends/README.md

@@ -85,6 +85,10 @@ For linux S390X cross compilation:
 
     sudo apt-get install g++-s390x-linux-gnu binutils-s390x-linux-gnu
 
+### Install the required dependencies: FreeBSD
+
+    pkg install bash
+
 ### Install the required dependencies: OpenBSD
 
     pkg_add bash gtar